I'm just giving pragmatic advice. The likelihood of cross save being implemented with PS doesn't seem very high, at least not anytime soon, to me as your progress/account is locked to your PSN ID, whereas on PC/Mobile you use/need a Mihoyo account no? Does Mihoyo even have the systems in place to link their account to PSN IDs? Wasn't long ago they messaged people in-game about how to safeguard their PC/mobile account. Will Sony allow that access?
Thus people recommending you upgrade your old console, and not strictly for Genshin only. Surely if you copped a PS4 at some point it's cause you're invested in some of their games, which will be better on PS5 and have sequels on PS5
As far as I know and have seen, the game on PS5 runs like how it does on PC. Difference is input method, but I know PC players that play mostly with controllers anyway