
It took me 90 years to go from 100 Romanismos Byz to 100 Latinitas Rome
So a warning for people who want to get both Belisar's Legacy (BL) and Mare Nostrum (MN) achievments.
My borders on Belisar's Legacy in 1688
My borders on Mare Nostrum in 1758
Both of them require pretty much the same borders but for BL you need to be Byz and have mended the schism using orthadoxy and for MN you need to be Rome and have >99 towards Latinitas.
I have done BL by 1688 and was 100 towards Romanismos societal value with greek primary culture and orthodox religion. I've kept all of that for stability during active conquest and coalition managment.
I however did not anticipate how hard and long would it be to change the value and how painful to change culture to latin.
Since BL states that you need to be Byz and have "Mended the Schism" modifier I didn't convert before finishing this achievment. But apparently even after converting to Hellenism this modifier still stays (which I didn't know). If I had to do this again but more efficiently I would convert both faith and culture as soon as I had pentrarchy and mended the Schism as well as having somewhat strong and stable economy and army.
I've also switched to a republic during long wait and got SPQR instead of just Rome :)