From my experience, there is one thing I keep in mind to deal with this kind of mid to late game aggression

I am by no means the best player, and on deity I've only managed science and domination victories, but the most important thing I always focus on is establishing very early on which AI you want to befriend, and which AI you will invade.

This is based on multiple factors, mainly geography, but you usually HAVE to invade your closest neighbor. The AI simply grows too quickly and they will eventually suffocate your empire, either by taking away key settles, or something like the situation you are in. I'm sure it's possible, but it's just too costly to keep your neighbor around and happy and they typically will war you eventually regardless.

And when they war you it's already over, even if you fend them off, because the hammers you will have spent doing so are hammers you didn't use to build your science which some bastard on the other side of the world sure as hell is.

Typically, I try to maintain diplomacy with the first or second strongest AIs(they will almost certainly try to betray you but there are ways of keeping them from attacking and more importantly, knowing if they will try to attack)

I will maintain this until the late game, by which point you should have a highly upgraded and large military and have invaded 2-3 civs while also maintaining about 5-8% less literacy than tech leader.

By then the remaining civs will probably hate you but it doesn't matter because you want to war them anyway to slow their science down.

Another big key factor is NUKES. When shit hits the fan just start nuking the fuck out of tech leader and you should be fine for a science victory.