Goku wasn't the first to reach SSJ2 and complete SSJB (at least in the anime).
Goku had already been defeated before Yajirobe cut his tail and Krillin hit him with the Spirit Bomb.
Where does this come from?
As for Ultra Instinct, it's a matter of talent and character. It's only natural that there's someone better than the others in a particular thing, sometimes protagonists are given this trait. If you really consider it "main character complex" or "plot armour" maybe you should try watching other genres.
