My guess is they'll go back through time using the quantum realm, and steal the infinity stones at key moments.
They'll have to go simultaneously so they don't override each other's timeline.
They'll grab the space stone, the mind stone, and strange will hand them the time stone. Those were all on earth.
They'll go back to the regular time, but nothing's changed since thanks has all the stones, the reality and time stones are forcing "his" timeline.
So the avengers fight him, but they have 3 stones. They finally get the gauntlet off, the 3 stones disappear, time corrects itself, everyone's undusted, but those who died the hard way stay dead.
They use the quantum realm to go to other realities and get their stones, and each stone is given to a different avenger, and they all take on thanks, stone to stone. The climax will be six avengers (the original six) using the stones to undo everything while thanos uses his to try and stop them, and the feedback shatters both sets, while warping reality permanently. People undusted but also history altered, powers altered, etc.
OG avengers more or less quit