Since the players are enjoying themselves, I would say the OP is kind of stuck. They have done the right things to encourage RP already. I would suggest a system like PDQ or Fate to encourage RP and to a lesser extent The One Ring. The thing is, if they are enjoying this game, its a bit mean to close it down and start another.
Having an interesting mechanical system often leads to mechanical play. PDQ in particular has such a simple system that there is no fun inherent in it. That sounds bad, but don't get me wrong, it encourages fun play, its just that there is no fun to be had tweaking numbers, looking for synergies or planning builds. In my experience, this means that people look for fun elsewhere, they build any character they want as they are all equally effective. Players feel unconstrained by the mechanics of the game in a hundred little ways.
Having an interesting mechanical system often leads to mechanical play. PDQ in particular has such a simple system that there is no fun inherent in it. That sounds bad, but don't get me wrong, it encourages fun play, its just that there is no fun to be had tweaking numbers, looking for synergies or planning builds. In my experience, this means that people look for fun elsewhere, they build any character they want as they are all equally effective. Players feel unconstrained by the mechanics of the game in a hundred little ways.