If you are really "brave", well motivated and with quite enough time to devote to your interest about that, here there's what imho is nearest to a proof. It's in lecture 8 videos while lecture 9 specialize the analysis to PoW blockchains, but you definitely need to watch (and understand) the whole course till there to understand... the many concepts needed -just to deal with it in a rigorous way- are introduced in a step by step way:

https://www.youtube.com/playlist?list=PLEGCF-WLh2RLOHv_xUGLqRts_9JxrckiA

just a quick note about what you have to expect: longest chain consensus attain PROBABILISTIC finality, even when formally described. The probabilistic nature comes from
the fact there isn't an a-priori knowledge of which node is an adversarial one, whichever leader election strategy (=way to choose who build the next block) it's chosen

Wish you a good learning work!