I believe the preposition that goes with becoming is to. If you use on, it means whatever is becoming can be removed easily. I can go with, “That dress is becoming on you,” because, although it sounds weird, a dress can be removed.
However, if you use on, you wind up with stinkers like, “That hair style is very becoming on you,” which indicates that she can just grab her hair and take it off her head anytime.
And you would never say, “He was disciplined for conduct unbecoming on an officer.” It sounds like his behavior was some fashion item he was wearing. You have to say, “He was disciplined for conduct unbecoming to an officer.”
I don’t agree that “suitable” is a true synonym for “becoming”.