I ended up wiping my graphics drivers using DDU (https://www.guru3d.com/download/display-driver-uninstaller-download/) and doing a clean install of the newest driver directly from AMD's website, rather than installing automatic driver updates through Windows. That appears to have fixed the problem, as BG3 now recognizes my correct display adapter just like my other Steam games do and runs smoothly as a result.

Cross-posting with https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=927724&#Post927724