It's a nice idea.
But let's put limit on one re roll only. for both 1/2 rerolls of greatweapon style and exploding dice.
Also lets take 65% hit rate and +3 str bonus on damage.
and use greatsword for 2d6+3 damage(reroll 1/2, reroll 6's for one extra damage dice(no rerolls))
So, out of 20 attacks 12 are hit for 11,33 damage on average and one crit of 19,66.
that is 7,78 damage per attack roll.
for exploding dice it's 13 attacks for 12,5 damage
that is 8,125 damage per attack roll
That is for relative high hit rate. Now for low hit rate of only 30%, +5 attack vs. 20 AC damage would be,
3,82 with normal critical and 3,75 damage for explosive dice.
and for medium attack hit rate of 50% it would be;
6,08 for regular crits and 6,25 for exploding dice.
Damage goes slightly towards exploding dice, but difference is marginal. It maybe offset by extra crit chance with advantage.
now for expanded crit range of 19-20 let's use exploding reroll of 5 and 6's on d6 damage die.
at 50% hit rate(8 hits, 2 crits) it's 6,5 damage per attack roll and for exploding dice it's 8 damage. That is a lot more. 1,5 damage on average is 23% more damage. So champions are out of the question here.