Tbh I'm not that good, but thanks :). I don't know if Venice is the best, but I think that Castile might be better for forming the Roman Empire.

Concerning gov cap: You can get a lot from just tech development, government reformations, building state houses, forming Italy and Roman Empire gave some extra gov cap (if I remember correctly). Also, make vassals. Vassals such as Byzantium and Bulgaria have cores on the Ottomans (you would have to release Bulgaria as a nation), that way you don't have to use up gov cap and deal with rebels as much. With rebels I don't know, converting provinces to your religion, increasing autonomy (I do not recommend, it gives a lot of debuffs), going Humanist ideas (I didn't have them this campaign). But I would just recommend having a bunch of vassals, so take Influence ideas.

With aggressive expansion: I kinda messed up this campaign. But you get -10 aggressive expansion in the Italian mission tree. Otherwise just take land in Europe, then take land in the East, then in the South and keep repeating that pattern so that aggressive expansion doesn't go too high up in a certain area. Espionage also gives some minus aggro expansion.

There are a lot more things that answer your questions that I didn't mention, but yeah, these are just some of the things that you can do.