After reading the description of Miles’s abilities, I’ve come to think that he doesn’t have a particular and clear “best build” but rather, that different builds will serve different purposes. There are a few other characters like that and I believe it is a good goal for developers when designing characters. The problem is that you basically have to “marry” forever one of your choices (unless you are a whale, who can have several copies of a character). I believe that allowing us to swap around covers (maybe for a small fee, say 200 Iso) would add an interesting layer of strategy that would come up often when you are looking to combine a character with characters that you usually don’t, because they are boosted. It’d also remove an unnecessary small amount of grief when you complete a character with the “wrong build”.
I do understand why you would think it’s is a bad idea for incomplete characters to swap covers around, but if you could add it as an option that unlocks once you have 13 covers of a character, it is hard to think of a downside. This would be an awesome feature to introduce if you are planning to buff old character at some point, if by using more modern design sensibilities you could make them so there’s not a single "correct’ build for them.