Wait, how are you getting five attacks? I get four--two from the attack action at 5th+ level, one for the opportunity attack (which you won't get every round; once someone's closed, and stays close, that ceases to be an option), and one for either the butt-end of the pole-arm or the GWM cleave. But where's the fifth coming from?
I meant if you're a barbarian in frenzy mode. Didn't I say that? Anyway, my point is that buff stacking through multi-attacks is fun, but there is no upside to dual wield in a game, aside from character concept, from a game balance perspective, because dual wielders can't ever benefit from GWM since there are no one-handed heavy weapons.
I think polearms either have to stop being heavy, or the feat needs to be changed to remove the bonus action attack. Or maybe consider the bonus action attack using the butt end isn't considered "heavy" so can't use the -5 / +10 aspect of GWM. But that's fiddly. Better to just remove the bonus action attack. It doesn't need to be there, and worse it steps on the dual wielder's toes too much. Actually, no, more like it steals their lunch money and locks them in their own locker.