For downsizing I use a would I choose this over this approach.
So for instance in Kemet vs Inis, Kemet must go.
Both are good and in an ideal world I would keep both and play often enough to get both to the table but the reality is that when I do play I would always choose inis.
Using this approach the current cull pile is merv, Kemet, Cthulhu wars duel and king is dead.
Limited space is my main winner and a honest ‘who will I play this with’ followed by ‘do they own this’ coupled with ‘stop making up imaginary situations’
For example, I’ll play Power Grid with anyone reasonably interested in a mid-complexity game. Plenty of my group own it but I’ll play it with interested no gamers too.
As a counter example, I don’t need a 7 player worker placement game that lasts under one hour
One of my other things to do is a would you rather with cold hard cash.
I saw Lords of Vegas go for £55 today and now that’s tempting.
I’ve not bought a game this year. There have been a couple of cheats (my wife bought it!). I’ve had games as gifts and have done a couple of Maths Trades.
Once it had got past 6 months I found I’ve been a lot more picky when listening to podcasts. Previously I’d have got home and started looking at Boardgameprices a soon as something piqued my interest, but I think the only game that I’ve really had a strong desire to buy after hearing a review recently is Polynesia.
BGA and TTS have helped loads because I’ve played a lot of games without buying them and then realised I can do without them. I think what I’m saying is that @Whistle_Pig has got me through the year!!
Selling a load was pretty cathartic as well.
My acquisition rate has lowered a lot since I changed my approach to it, so I think it is good you are doing it too. And you should not worry too much to share your success and failures over here, I am pretty sure some of us (I will, for sure) find it helpful.
What helped me loads was stopping notifications from second hand Facebook pages. And KS, Patreon, etc away from my phone. I mainly watch anything to do with board games now only on weekends.
My only comment would be, with Christmas coming, perhaps tell your loved ones the unbelievable thing: “I don’t want board games this year”. For it is a bad time to downsize, right now, with all the mistletoe around…
This reminds me of our discussion back in 2020: Gaming Goals for 2021
I have not bought even one board game in 2021 though I asked for the Crew 2 as Christmas present.
So I will repeat the things which helped me and put them maybe in better words:
Don’t listen, read or watch (almost) any board game content. I rarely meet new games, I never have a feeling of missing out. The few new games I meet, I either don’t need in my life / collection or they are so good I buy them. But that is a ridiculous low number. 1 in 2021 (expecting my Christmas present).
Ignore Kickstarters completely. You are excited about a game, but till it arrives at your door step, there is a good chance your excitement isn’t as high anymore or you have a better game for that spot or your circumstances changed and you are not interested anymore. And even if all of that is not true, you will still be able to buy the game right when you want to play it.
Don’t get expansions. Like Admiral Ackbar likes to say “It’s a trap!”. There are very very few games for which it is worthwile to get expansions. There are two reasons in my opinion. First, we (owners of big collections) don’t play the base games often enough and second, we (owners of big collections) tend to introduce new people to board games and it is most of the times a bad idea to include expansions if someone is unfamiliar with the base game. A third reason just came to mind: New games are coming in, occupying our interest, so even expansions fail to keep up the interest for the old timers sometimes.
Just think about, how many of your games you have not played or only once. I am sure, for most of us this number is incredible high. Pile of shame is the right term for it and tbh I feel it for boardgames more than for pc games because they tend to be more expensive, take away more space and are harder to utilize.
It helped me to just accept that I have a fine collection with a ton of games and enough of them we never played. No reason at all to get even more. (At least not in huge numbers). I know that’s easier said than done, but still. I reached the point where I just feel saturated. It is a nice place 
This has an extra bonus of the feeling of adding time to your day. I rarely consume video reviews because there is enough on Netflix to occupy that time.
To curb new acquisitions, you could set yourself a tough budget for the year. If you have financial constraints on what you can spend, you may be more critical on the purchase.
Plus you can then play around with little houserules by justify a budget increase by selling a game or two, which again will get you looking at what you already have with a more critical eye.
A few things I’ve found moderately effective in reducing the rate at which I buy games:
having a wishlist. I put things on my BGG wishlist when I’m tempted to buy them, and then forget about them. I realise that having a wishlist has the opposite effect for some people, so YMMV.
having a “hobbies” budget for games, plants, and art and craft supplies. I buy my convention tickets out of this budget too, so I have an incentive to not spend all of it every month.
doing my “play all the unplayed games” challenge and recording my progress here. I haven’t completely succeeded in not buying new games when I have unplayed ones, but I have reduced my pile of unplayed games to 9 from around 50
unsubscribing from mailing lists/notifications from game shops and Kickstarter
finding a game shop that does retail pledges for Kickstarters. I’m much less likely to impulsively back a Kickstarter if I have a good idea it’ll be available later. Putting things off is the key to success 
I don’t know if you’ll find any of this will work for you. My brain is a combination of impulsive and extremely distractible, which is probably why the delaying tactics are effective!
I’m the opposite! I’ve not watched much on Netflix recently because I’ve got so much YouTube (or similar) content to watch.
Thanks everyone for your suggestions and ideas. I am definitely taking some of those.
As stated elsewhere I have already put a few games up for sale. First one is already sold (Loot for Lima—I‘d rather keep Cryptid & Treasure Island).
I am using geekgroup app to tag games with notes for selling or keeping.
There are about a dozen games marked as nostalgia that I am unable to let go of even though I anticipate I will not be playing them again.
I have also grouped games by weight and figured out that I have a larger share of light-weight and medium-light games than I would like to. So that‘s where some more of the cutting is going to happen.
It is not too hard to cull games—at the level I am at right now there are plenty of candidates.
The hardest part is not buying new games. I have hopes that once I have cut the thing down to size that it will make me dislike blowing it all up again—the weightloss comparison by @lalunaverde is not too far of the mark.
Limiting exposure to the latest games is not easy, I do not watch a lot of reviews besides SUSD & NPI and I only manage to listen to podcasts on occasion but I read too much. I have tagged all my games with „sources“ for a while now and I can easily find out where I should tread more carefully…
Budgeting is what I tried at the start of the year. Maybe a second attempt might get me there. And not a budget of 1 game but rather a monetary one.
Wishlist keeping usually only makes me buy more games. Maybe I should go the „fire and forget“ route. For a while I marked games I was interested in simply as „want to play“ where they stay out of sight for the most part.
On the budget side of things I find it helpful to keep my hobbies allowance in a separate bank account so I can easily see what’s left, and to make it harder to spend accidentally.
For any game you can’t get played, spend hundreds of hours implementing it digitally so you can play it, and thereby justify the purchase.
I know, that’s not exactly helpful advice, but it approximates the lens through which I view game purchases.
More generally, perhaps, I find that playing games online tends to eliminate my desire to buy the game. Recent-ish examples include Beyond the Sun, Antiquity, Lost ruins of Arnak… if you can get past the honeymoon phase and play a new game quite a lot, you may well find there’s no reason to own the physical object. I can also see this backfiring, but at the very least, having to find the time to play any potential purchase a half a dozen times will limit the rate of acquisition!
So by sources my games were recommended to my by
For many recent purchases there are multiple sources. I first see a game on BGG, then hear about it on a podcast, somebody here plays it and then Spacebiff writes a review and NPI makes a video… and then game bought.
I usually ask myself “Why you want to buy this game?” no matter the answer, I ask myself: “Could another game I have fulfill the same purpose?” Mostly the answer is yes. So I either get rid of the the one I own first in order to buy the new game, or don’t buy it at all.
Reviews aren’t a problem, at this point they are more entertainment than anything else to me. Going into a shop without buying is harder for me.
Edit: One thing, that kinda helped me was the Friendless metric and the continious friendless metric, which is made to show you how good you use your collection:
From the website: War Table | Extended Stats
To calculate the Friendless metric, make a list of all of your games in descending order of how many times you’ve played them. Scrabble with 173 plays comes first, Triominos with 0 plays comes last. For each game at the beginning with 10 or more plays, we consider that game to have earned its keep. Even better, having played one game that many times, you’re forgiven one at the other end which you haven’t played so much. So for playing Scrabble so many times we’ll forgive you for never playing Triominos. People who play games tend to accumulate them, so we’ll assume that was a gift. Then, we look at how many times you’ve played the last game remaining on the list… and that’s your Friendless metric. The higher the better.
Most bggeeks have a value of zero, so I extended the definition to go into negatives. A value of -x tells you that to get to a Friendless metric of 1 you need to play x more of your games that you’ve never played.
First play of Imperium Classics (live): if there had been a game vendor there I’d have bought it on the spot.
Second play (TTS): yeah I enjoyed that but maybe I won’t rush to buy.
Oh noes. It wasn‘t my intention to…
So I should address that…
I am not opposed to getting new games. This is more about stopping myself from continuously buying more games. Presents or trades are fine.
Please, Santa, don‘t misunderstand this one -.-
The wishlist advice is great. It’s good that bgg wishlist comes in tiers.
One thing I ask myself about buying a game is: can I just get it later? Preferably, a year later. Idk how the 2nd hand market looks in Germany but Im sure you can forgo the hotness in the meantime. Retail price also gets slashed after a year, since our hobby is driven strongly by the hype cycle
Why not make a rule about no games released from this year and last year? This stops you getting sucked into the hype. Then, you have to make a strong justification if you want to break that rule.
I only buy games in SPIEL and UKGE if i have tried the demo. Con stores are usually pricier than retail stores
- SUSD (50+)
I dont watch their vids and listen to their podcast anymore. Their vids hype you up and I feel that their podcast is a waste of time, except Tom and Ava sometimes makes some spicy takes. Spicy takes that Im happy to skip nowadays, alas. I still kept my monthly donation to them tho.
- Crowdfunding (30+)
This is easy for me now since the EU (and the UK) crackdown on their VAT dodging.
- Spacebiff (20+)
We have similar taste, Yash. But I never got into Spacebiff. His podcast is great tho
- FLGS (30+, over the years though and have a bad keeper quota)
Why not get games from Geekmarket or eBay Kleinanzeigen in the meantime? 
- myself
Same. Being self reliant on finding info of new releases myself has its drawbacks.