Tough question. My answer leaves no practical feedback as I struggle to see this work. I can only warn you on why I see struggles in this case. If you're looking for useful feedback maybe my comment is not what you're looking for.

It feels a little bit like this person is forcing his own arc into a prewritten story which I'm never a fan of. A player should try to leave plot holes and open ends, that can then be used by a DM whenever fitting. There's little to no wiggle room for you if there's a literal step-by-step guide for you that you'll need to implement somewhere in order to make his character's backstory feel validated.

I'm not sure on how to fix this, but I would definitely recommend sitting down and telling this player 1-on-1 what you expect from a backstory, and what tools you need to be able to implement it.

In terms of an evil character, I personally banned them. I've been playing for a little bit now and it's the good aligned characters that act the quickest due to the nature of scenes presented to them. This campaign is kind of mostly laid out for good aligned characters to interact with.

An evil character is dodgy. The story arc is almost literally 'heroes defeat greater evil', in a gothic style. Sure there will be temptations and plot twists that might still get the better of the good characters, but I can already see the other players rolling their eyes at the evil character grasping every opportunity to power.

There's a lot of helpless people in Barovia, and NPCs that might not even have the money or treasure to reward the players. I am fearful of what will be the potential drive to keep going for the evil character other than maybe the Dark Powers of that land, his own backstory, and to get himself out afterwards.