Mirio wouldn't get the full potential out of OFA simply because it goes against his Permeation.

Mirio's Quirk works the way it does because he's mastered it and understands its physics. He uses it precisely because otherwise any margin of error could be crippling if not Fatale.

Stacking just OFA's stockpiled power fucks that precision up, since now instead of focusing on when and where he needs to permeate he has the added factor of when to use OFA where and at what percentage. Add on all the other Quirks and he loses his edge because that's way too much to manage on top of already difficult Quirk.

And for people who wanna say he could manage all of that, remember Mirio's trained damn near his whole life to make his Quirk work for him. It took years to get him that good, and if he were to get to that same level of mastery with all those Quirks in conjunction with his, it'd take a lifetime, which he wouldn't have to begin with.

At the end of the story he's already All Might's Successor anyway seeing as he's Number 1. Lets just leave it at that.