I have Dwarf Fortress on my wishlist and while it's cheap to pick up...yeah that looks like X4 levels of complexity but in 2d. Not sure if I'll ever be ready for that, haha.
As someone who loves it, It's less a game and more a story generator. Until the company was hired to do the nicer graphics and interface for the Steam version it was a math PhD and his brother programming it as a work-in-progress complete fantasy world simulator. It still is but now it's prettier. It feels very comfortable to call it the most complex game on Steam. Rimworld and Minecraft among others took direct inspiration from it, he's been working on it awhile.
Famous patch notes include fixing cats dying from alcohol poisoning because they walked through a puddle of beer before cleaning themselves, egg yolk and egg white having different fluid densities, and nerfing mer-people farming because that's just disturbing.