GWM for the power attack? Nothing kills dpr faster than reducing chance to hit, as a miss does 0 damage. Other than the power attack option, most of GWM is wrapped up in frenzy already, frenzy already gives you an extra attack as a bonus action just because you are frenzied. Power attack is a good way to miss attacks that should have hit.
As far as balance goes, i've been in published adventures with only 2 combats and a horde of skill challenges. Many of the combats lasting 5-6 rounds, or longer in some cases as running into fights you aren't supposed to win does happen. I think the PHB puts it best for beserkers, "For some barbarians, rage is a means to an end-—that
end being violence." I find it kinda ironic people over thinking beserkers, a class that doesn't do much thinking. The point is to blindly murder anything that gets in your way. Frenzy can be used every time you rage, there are also spells that remove a level of exhaustion. see greater restoration. Some one mentioned being hit with CC when you frenzy, could always wait until after the spells are used before frenzing, or accepting the fact that counter play does exist, being CC'd while using concentration spells messes up some builds pretty badly, rangers come to mind.
The berserker is all about dealing damage, while the totem path gives more utility, the berserker does nothing but murder things. If you can roll higher enough to merit power attacks sure GWM is nice, but taking a feat you can get half of for free seems silly, other feats might be better suited, Charger gives the damage boost without the attack penalty, mage slayer to make those mages reconsider their CCing ways, magic initiate gives some interesting options, martial adept lets you do cute things, savage attacker lets you reroll a damage dice and take either result, sentinel to make sure your target doesn't get away from you. Tavern brawler and grappler can do cute things too. But both GWM and polearm master have half their components useless whenever you frenzy, and if your not wanting to frenzy then you might as well pick a different build. until the third level of exhaustion it's mildly annoying at worst, meaningless at best, and you don't take the exhaustion until after the rage is over. Which unless you get knocked out or don't get hit by anything or don't attack anything yourself could go on for the full minute, about 10 combat rounds IIRC. Do remember to call out your DM for meta gaming too. Not everyone is going to know to leave the barbarian alone to stop it's rage. You could theoretically set a touch to burn you for 1 damage per round so that the rage doesn't end until it is put out or the minute ends.
A berserker who isn't at 3 or more exhaustion at the end of an adventure isn't using their full potential. As the chance to take as many days of rest between adventures as you want is always an option. besides it's only rank 5 and 6 that can't be managed during an adventure.