>>2036846
also got stuck around 700 pop for a while, what got me out of stagnation was giving my citizens some clothes/furniture, the fulfillment bonus from clothes alone goes up to +7 which is pretty damn good. Unless you're already mass producing fabric you're probably better off buying the clothes directly, though it'll be quite expensive regardless
As for the building decay, the reasons are(usually) lack of materials, the building being out of the janitor building range, not enough jannies, or you losing the tech necessary to keep that building running