I realize this a discussion about balance, so feeling something is overpowered is not off-limits, but it does strike me as funny that you feel GWM is OP, while not Polearm Mastery. Also GWM is better in the hands of Berserker than Polearm Mastery is (this could be debated), so GWM needs to be in the discussion if we are talking about balance.
When I was making a fighter, I wanted to really get a sense for weapon balance, in this process I had to look at feats involved with each weapon choice. The feats that I looked at, were GWM, Shield Master, Polearm Master, and Dual Wielder. I also considered a combination of GWM and Polearm Master. I ran average outgoing damage and incoming damage, based on my group makeup (melee rogue and paly with GW). At lvls 5 and 10 (and just a little at 20) with both advantage and no advantage, and also burst rounds vs non burst rounds. My conclusion was that GWF and Polearm Master were very good, but definitely in the same ballpark with sword and shield with Shield Master. The reason this is relevant, is that in my numbers the real equalizer is not Polearm Master, it is GWM. I realize that you find it OP, but it is this feat that makes the Berserker on par with the Totem Warrior. Any discussion about balance between these 2 paths needs to look at GWM.