Only fellow I'm aware of that's able to do slide mill is Langdon Tactical. It'll cost you a pretty penny at $325 but they're the o my ones I'm aware of that can or will do it.
I would go with Wright Armory. The LTT cut is good but their cerakote finish isn't great. It's really thick and shiny rather than matte like the original nitrided finish. They also coat the entire slide including the polished underside of the slide which results in a high friction mess after less than 100 rounds.
Edit:
Here is a photo for comparison. The LTT re-finished slide is on the left and the factory slide is on the right.
I have two slides at Wright Armory now. Not cheap but they use QPQ Black Nitride for slide refinish which is closer to factory and preferred over Cerakote.