JustBen
Prince
Yes I've restarted a game with Isabella while kinda ignoring the Religious tech path... and I've went trought an outstanding start ! At some point, I was the tech leader by some good margin and then a GP poped and I bulbed Theo, so I founded Christianity... But later, the next GP I got, I didn't even found the Church because there was only 3 cities with this religion and I didn't want to bother spreading it, i felt it was a big waste of time and hammer !
Perfect! Sounds like a strong standard game that is well preparing you for Immortal.
Now I've went trought a war with my neighbor, who I erradicated, but I think it compromised my lead in tech. ... should I try to bother him with espionnage while keeping good relations or should I kick him in the face ?
This is typical at Emperor and up. A well-chosen war is almost literally mortgaging your economy. Usually standard econ play can fix this -- courthouses, libraries, specialists, and building Wealth. If your opponent is strong enough, espionage may be called for.
Ideally you trash your economy to build a huge army ahead of time, spend your savings on the invasion, and recover while you assimilate your conquests. Less stressful wars require less pain beforehand, and less time to recover.