Some of your math isn't correct.
2d6 with GWF is 8.33. It's AVERAGE(3.5,3.5,3,4,5,6)*2 in excel/google docs.
1d10 with GWF is 6.3. AVERAGE(5.5,5.5,3,4,5,6,7,8,9,10)
ehm nope.
when you roll a 1 or a two you can reroll BUT you must keep the second value.
This means that you must also average the 1 and the 2 with the die's average.
1d4 becomes ((1+2.5)/2+(2+2.5)/2+3+4)/4
1d6 becomes ((1+3.5)/2+(2+3.5)/2+3+4+5+6)/6
1d8 becomes ((1+4.5)/2+(2+4.5)/2+3+4+5+6+7+8)/8
etcetera.
TWF actually benefits more from boosting dex than taking dual wielder. +1 to hit and damage is worth more than +2 to dmg.
You are mistaken.
Dual Wielder provides +1 AC, removes the mandatory "light" quality to use TWF and allows you to sheathe/unsheathe both weapons at the same time. Second and third are quite compelling, especially the second one, if you wanna perform properly as a dual wielder.
IF you stack dex with a feat, you'd better do so when you have an odd value and need to even it
GWM's can sometimes use a bonus action. At level 4+ they do more DPR than TWF.
Polearms can always use a bonus action and even have a reaction.
GWM provides you the formerly known as Cleave feature, which is purely situational. When you theorycraft, you'd better stick to consistent mechanics
Shillelagh increases the die value of the quarterstaff to d8, it makes no change to the "back hand" of Polearm Master, which is d4 and always have been. The only thing affecting the backhand is that it is Magic Damage rather than bludgeoning damage, because all the effects of which the main attack benefit, are transferred to the back hand as well, i.e. if the polearm is magic +3, the backhand will be magic +3 as well.
You're still saying TWF is not good at 1-4. That's actually the only point where it is competitive.
I've done the full math on
DPR of Classes if you w ant to see it. TWF loses 4+
It was never competitive to a min maxer's eyes. To a roleplayers it is because they often don't give a frack if a stle performs better than the others, they just go for the badassery (something I respect much)