kembreg: I think that rector is your best choice. It’s used with helmsmen/pilots (on ships) and, possibly, with chariot drivers. (Actually, I’ve forgotten the exact term, but it would be easy to find in the fifth book of the Aeneid.)
Circumlocutions like ille qui regit will easily tire unless they are used rarely. So if it’s once or twice, I think ille qui regit is fine, but if the bus driver’s a major character, it might be a problem.