I play at 900p60. Turn literally everything to low or off except textures at medium. Enable the AMD upscaling to the highest quality setting (forget what it’s called). Be sure to turn off Antialiasing (don’t really need it at high resolutions) and God rays. Turn off all optional things but those two are the most important. Also, if BG3 is installed to an SD card, then enable slow HDD mode.
It still stutters a little when transitioning to cut scenes, but I believe that exists in all PC versions.
Edit: And I have made it (what I think is) mostly through Act 2. I’ve also hosted an online session with my friend (who also plays on Steam Deck using my settings) and my husband (gaming laptop) with no issues.