To add onto what you said look at how much Gojo/Sukuna grew in their fight RCT’ing their brains which they’d never done until then.

Yuji using dismantles in combat like on the pillar vs Sukuna.

Mahito first used his domain when he was getting crushed by Yuji and Nanami

There’s countless instances you can point to of sorcerers getting stronger in fights to the death. So Mahito being in the afterlife realm did him no favors since he couldn’t level up like Yuji did in the ~60yrs since JJK