Finally I had the time to try up this, if I may add, great scenario. I would appreciate some advice as my northern plebs don't seem to want to stay too long ... I wonder why, with all those goths roaming around ;)
I´m glad you enjoyed my scenario, this one was intended to be tougher than Caesarea and Athenae.
On your issue with devolving: you took the patrician route, but that´s really just an option in this case. I made 2 test runs myself, one with palaces, one with GI as the top class. In that route with palaces, I ended with terrible worker shortage, as you have too anyway. That´s why I lowered the prosperity requirement. It would require much more planning (and probably less palaces). First of all, you are overproducing food. I do not understand that number of wheat farms, as most of your people live in GI´s. They just soak up lots of workers and space: some of those devolition problems are caused by lack of library acess, some by lack of furniture. Both can be solved, but then, palaces devolve too. Of course, your city is a good one (you fulfilled the winning criteria), but to maintain a stable one, I would go for another setup: maybe to leave some plebs in medium insulae, or bigger population (as if 7000 wasn´t hard enough)...