ProtonTricks may be the app for your problem. It can run Winetricks specifically targeting the prefixes that Steam sets up for each individual game. Soo, (I do not have BG3) run Protontricks, pick the game, wait, find that Winetricks starts off at a really awkward point in its UI, pick add an application, cancel, find yourself at a better menu, choose “Install a Windows DLL or component”, and check off the appropriate dot net version.