For what it’s worth — I always use the argument that Mary Sue-ism is not necessarily bad -depending on the context-. It’s hard to argue that Twilight and later 50 Shades of Grey weren’t popular because they were 100% wish fulfillment for a lot of its audience. In some movies (mostly the old ones), James Bond could also be a Gary Stu but to his credit, he was a glamorized spy in post-WW2 world where most young men were going on to work boring office jobs and navigate a new world order and so the escapism ended up being really enticing.
One of my favorite characters from Fire Emblem, Ephraim, is a fairly big Gary Stu but his story deconstructs how all his “awesome qualities” don’t help with the situation at hand and actually were very damaging to his interpersonal relationships.
Rey’s problem is that Star Wars is well… Star Wars. Which is to say an already-established universe full of characters that each fulfill their own story niche.
Except now they don’t have to because Rey can do it all and then some. Which doesn’t work at all, nor does her story even acknowledge the Sue-ness to do something new with it.