Just in case anyone is still having trouble with this, solution: Find your DoS2 game files
under 'Steam\steamapps\common\Divinity Original Sin 2\DefEd\bin' and launch the Definitive Edition '.exe' file located there.
Answer was kinda simple, but to be honest, I didn't expect a different .exe for both the versions, which to be fair I should have checked for a lot sooner.
