For your city builder, I suggest checking out "timberborn" on Steam. Its a colony sim where you are a cohort of beavers. After you get the hang of the core mechanics, the emphasis is on larger-scale constructions. I certainly feel that in certain maps, I need to introduce temporary workarounds to scale up my bigger works.

It lacks some of your "buildings decay" ideas, though. So not quite 1-1 with what you're suggesting.