When installing shaders, people often use "OptiFine" or "Iris", but

Personally, I'd recommend Iris. I tried both on 1.21.1, but OptiFine kept crashing my game. There seemed to be a lot of talk about poor compatibility, but it really does seem unstable. At the time, the only other mods I had installed were... "Korean Language Pack Mod" and "Minimap Mod". It wasn't those causing the issues, though.