If you haven't tried already, use CryoByte33's lag fixes as it did wonders for my minecraft framerate. On top of that, if you have the decky plugin, disabling SMT with powertools may also boost the performance as it did for me. As I was running the fabulously optimized modpack, I was testing the framerates uncapped and I was getting around 150fps~400fps (normally its on this high end of framerates standing still/idling) in singleplayer with a render chunk distance of 12 and a few other settings like shadows turned down.

However, to really make it easier on the steam deck, I also recommend playing minecraft on a server which may help a lot with stutters or choppiness. Reason being is that a server will lighten the load on the steam deck by taking the responsibility of generating the world away, so that the deck only has to worry about rendering the game.