I know this might feel a little off topic, but I think it would be worth mentioning here as another way of handling stackable (dis)advantage in PbtA.
The BitD style of rolling reminds me quite a lot of how FU RPG works.
My main issue with stacking advantage is how quickly we get to Full Success territory (or Yes, and… in FU). Again, this will depend on the style and feel of your game, maybe you are shooting for players not having any complications a lot of the time.
It seems that playing with Fail/Complication/Success target values remedies this a bit, but if you want more granularity in a stacking advantage, Earthdawn: Age of Legend (a more indie approach to the world of Earthdawn, based on FU RPG) does it in an interesting way.
Instead of advantage/disadvantage giving you an extra d6, it gives a Fudge die. If you have advantages and disadvantages at the same time, they cancel each other out (so 2 advantage and 1 disadvantage gives you total of 1 advantage).
Then you roll your d6 and amount of Fudge dice equal to your remaining advantage or disadvantage. With advantage, each + gives you +1 to what your rolled on the d6. With disadvantage, each - gives -1.
I haven’t played this version of FU / Earthdawn to give feedback myself, but according to the game text, each (dis)advantage changes the value of success/failure by roughly 10%.