The reason why he has potential caps on all his power (Ex: Quincy Lv: 1/150) is because every being has a max limit imposed by the Soul King.
It's why Aizen wanted to break the barrier between hollows and shinigamis in the first place.
You can also increase your potential by breaking your limits and evolving.
(Either pull an Ichigo and go Mugetsu, or do what everyone else did in the 1000 blood wars.)
He had maxed out his personal growth as a shinigami (not including Bankai, and yes I'm choosing the "Aizen neglected Kyoka Suigetsu" route so I don't have to make up a theoretical Bankai for Aizen) so he needed to evolve beyond shinigami.
And different people have different "Max potential", also even if someone's unlocked more of their potential then someone else, that doesn't mean they are stronger.
Ex: Chad unlocked [30/70] vs Orihime's [40/75])
Chad is stronger, but Orihime is far more versatile.
Heck, a level 1 could even beat a level 40, as the levels represented how much your powers can advance/grow.
Also. Skills have no level cap, they are just really hard to level up, especially after 100.
But here are some base lines(kinda)...
1-49 = Average Shinigami (Most of the time)
50-99 = Lieutenant Class (Most of the time)
100-150 = Captains (Most of the time)
300+ = Aizen Post Hōgyoku
500+ = Post Dangai Ichigo
600+ = Mugetsu
700+ = Soulking and Yhwach
So just like Dragon Ball, Levels are bullshit 🤷♂️.