You can do that with Steam too, I know because I’m doing it. I have dual boot, I use Windows very rarely (I play on Linux) so Steam is not installed on it at all, I copied BG3 on it to try out mods because Mod Manager doesn’t work on wine for me.
I can assure you the game works perfectly fine without Steam.
With the steam version it’ll try to launch / connect to the local installed steam at startup
As you sure you’re using the right exe? bg3_dx11.exe and not some launcher?