Hi Aenima,
there are some buildings that "help" economy and happiness (as Food Trade tree, Burials tree), but even different buildings sequence that help each one together, as Roads - Market - Farms (a building giving happiness only after Maket is useful
...).
Exterminating population in newly captured towns (with different culture) destroying all possible different culture buildings (carefullly anyway
...), and recruiting as many Western Peasants as possible can help you to keep people at least with blue face, while building Burials - Metal Trade - Food Trade - Horse Trading - Academy - Christian Church. Can be long and risky, but it worked for me in my previous 434AD Campaign.
Finally in 434AD Campaign I chosed which land to keep and which land to leave, looking in the map the easier lands to defend (mountains, rivers...)
ayeaye Sir
PS it's the right time to ask for "modification", while Rio is patching
...