That's not 5E D&D, and BG3 is supposed to be 'the definitive 5E game'. If you want your thief to use magic routinely, then you can take the arcane trickster archetype, which I have used with Astarion on multiple playthroughs. Or multiclass with a magic using class when that becomes available - no need to overturn the existing rules/balance.
Also, lock-picking is not tied to the rogue class in 5E, so the current implementation is correct in that respect.
