Actually, isn't the "Exiled from the Party" genre a lot more satisfying when the protagonist was great from the start, and their old party just didn't realize it?
If the protagonist gets a random power-up the day after being thrown out, that's just luck, isn't it?
Like if I were fired and then I won $100 000 000 on the lottery next day, it's nice, but not really relatable.
That kind of event is better in a "Betrayed by the Party" scenario, where you were about to be killed as they threw you to the monsters, but a fluke upset their sinister plan and gave you the power to fight back against certain doom. There, the point is the revenge, a fantasy where the bad guy loses for once.