Now that you got rid of the leveling system, can you make some other way to increase psi storage and regen?

Leveling wasn't tied to maximum psi storage for about 2 years - release 1.1-60 (Nov 2018) made everyone start with 5000.

Ok, fair enough, but still. Is it possible for you to add something to increase psi storage and energy?

It's possible for add-ons to increase the maximum psi amount and psi regeneration (Psio does the latter). There are no plans at this moment to add this to the base mod.