Suggest a point based system or mechanic that address my issue with point buy

[Morty]

Unfortunately, I think your premise is flawed. I'm not sure how being given a round peg for your round hole - that is, a magic-using thief - is worse than being given several square pegs and allowed to decide which of them to try to force into the round hole, which is what D&D does with most character concepts that are remotely original. And as Knaight said, most point-buy systems will allow you a great deal of freedom within the bounds of your concept, which in this case is "magic-using thief".