I've been crunching numbers this evening and it seems pretty dang optimal for melee damage in 2024.
MA, GWM, Graze (7+4+3)+(7+4+3)+(3.5+4) = 35.5*.6 + .4*4*2 = 24.5
I'd estimate the GWM bonus attack procs about 35% of the time for +2.275 DPR = 26.8
GWM, PAM, Graze (5.5+4+3)+(5.5+4+3)+(2.5+4+3) = 34.5*.6 + .4*4*3 = 25.5
I'd estimate the GWM bonus attack procs about 35% of the time for +1 DPR = 26.5
The Battlemaster manuevers will slightly favor the non-PAM build (+1.5 damage pre accuracy adjustment per maneuver divided by the number of rounds in the period, probably yields about a +.5 DPR increase). However, the reaction attack is the big variable and since the DPR is going to be so close it won't take a very high proc rate to push this over. Something like 2 reaction attacks per short rest from PAM is likely enough to put the PAM version in the lead ever so slightly.
I don't think it is close to optimal even on a single class build and the numbers above are not correct.
My understanding is you do not get a GWM damage bonus on either your bonus action attack or your reaction attack. What I have read of the GWM feat is that it only applies to the attacks that occur using the attack action. So the PAM bonus action on PAM-GWM is 2.5+4, not 2.5+4+3 and you also do not get GWM on your reaction attack or your hew attack.
This is a big reason GWM-PAM is outclassed in general by other styles IMO, not just in this example. IF you got GWM with all your attacks it would be way different.
RAW I think the highest damage in a single class before level 11 RAW is going to involve a lot of weapon switching, dropping and drawing different weapons with the Dual wielding feat. I think it is going to be something like: Attack with Greataxe-Cleave-Drop great axe-draw scimitar-draw and attack with shortsword (vex), Nick with scimitar (nick)- drop both weapons-Dual Wielding bonus action draw and attack with a Versatile Longsword or Warhammer.
This crazyness aside, I think just basic light weapons with nick and the dual wielder feat is going to beat GWM-PAM by quite a bit.
What feat are you choosing? I'm presuming +2 Str to max out your str at 20?
Then use the exact same feats GWM-PAM. You are just only using the PAM for the reaction since martial arts does more damage than the PAM bonus action attack.
Switching from a 2d6 to a d10 weapon drops damage by 3DPR drop in damage from level 6 to level 7 to get the reaction attack, it also drops damage on your Hew attack when you get it.
Maybe that is worth it, maybe not but this makes it an easy comparison - same weapon, same strength, same opportunities and number of attacks and slightly more damage
Should be back to virtually the same situation as at level 6 as the PAM gets a feat like sentinel, which in addition to putting str at 20 also has an amazing tactical effect and combo with PAM, as well as providing a few more reaction attacks that the MA version does not get.
They both can get Sentinel, and the multiclass is still doing more damage with their bonus action.
What feat are you taking here to increase your damage, i'm not really seeing any offensive ones that really benefit your character at this point? Match sentinel feat maybe so you can at least assume more equal reaction attacks? I really don't know what else you might choose?
What I used in the earlier post was GWM only with a better weapon meaning more damage. I think in general PAM-GWM with a d10 weapon is going to be substantially worse than GWM with a 2d6 weapon and martial arts and I can show clearly that the latter is doing more damage on your turn (over 4 more DPR) with or without sentinel.
But whether it is outright better relies heavily on the value and frequency of your PAM reaction attacks as compared to that extra damage you do on your turn and the frequency of Sentinel attacks. More Sentinel attacks or fewer reactions overall would favor not using a pole arm (even if with the feat)