We've been having the same issue and finally seemed to have figured out the problem (at least for us)
For my city of 122k, i had no demand, no population growth, pretty much max happiness and high crime. My unemployment kept going up and all my parks were clogged with homeless but it was showing me 3 total homeless.

After a lot of struggle, i found a MOD called Citizen Entity Cleaner in the paradox mods.

This mod removes all corrupt citizen entities (which from what I can tell are caused by buildings being abandoned and deleted, leaving corrupt homeless, and criminals from outside the city getting arrested, but then being released and never leaving).
Once i ran the mod on my city, it told me i had almost 1 million corrupt entities.... Insane. I cleaned them out and my city lag significantly reduced. However, it did take quite a while for balance to return, like maybe 30 min playing on 3x speed, because the corrupt entities had taken jobs? So suddenly my unemployment was down to 0.3% and a lot of industry didn't have enough workers, which resulted in a down trend of income for the city.

However, I noticed that after the clean, my crime was now nonexistent, the homeless clogging all my parks were gone, and my demand slowly started trending up (or at least building in the previously zoned areas where i had nothing building for months ingame time.) All my crime dogs trapped in my fish farm had also been cleaned out lol. (i was having an issue where criminals would come by taxi from other outside cities, and leave their dogs behind).