I just thought of something...
When going back in time why not just go back to before Thanos destroyed the stones and take him on for all 6? They already knew he was weakened from doing the snap. Then when they go to the present to snap everyone back and then return the stones to the past the Avengers of that alternate reality will have an extremely easy time of bringing their own people back because they’ll show up to an already dead Thanos and the stones just sitting there.
MOVIE LOGIC!!!
When they go back to replace the stones in the timeline before Thanos destroys the stone (the past can't be changed) he could use the Time Stone to jump further into the future and undo the unsnappening, or, as he threatened, to keep people from undoing the snap, destroy the universe to start over.