I have played Architects of the West Kingdom solo but from what you write about Terraforming Mars, I would recommend Paladins of the West Kingdom over Architects. The solo mode in the second game works better also the complexity of Paladins is closer to TM in my opinion. (edit: but Architects is better if you ever want to play with a group, as its somewhat light-weight worker placement is much easier to sell to people than the giant puzzle that is Paladins-small box doesn’t mean small game)
I would second Nusfjord, the solo is awesome.
If you like empire building, you might want to take a look at the Imperial Settlers games, as far as I remember (it’s been a while, since the game didn’t work out for us as multiplayer, I gave it away) the solo mode was decent enough.
Gaia Project has a solo mode that is supported by https://myautoma.github.io/ I’ve only played the solo without the web though and that was a bit of a “figure out where what goes” thing but it worked better than playing Terra Mystica against myself. I’ve played the web-automa for Wingspan several times and must say that it streamlines the process extremely and makes the solo for Wingspan quite smooth.
I have not tried the solo for Viticulture (also on myautoma) but I have heard good things about it.
If anyone has played Tapestry solo (just saw it on myautoma), I’d like to hear about that.