I'd like to add in, if I may, rather than start another thread. Hopefully the devs will read what y'all have said in this thread; some good constructive criticism.
I'd like to float a couple other ideas I feel are really important right now for pure psi:
--Draining psi and psi reserve when switching abilities has to go away. Not because we posting here can't work with it, but because it disproportionately punishes new and less experienced players. If you want a metaphor for it, it's as though all of a sudden when reloading guns all your previous bullets disappeared forever. The idea behind the innervation system is that full psi users will need to limit themselves per encounter, while still being able to enjoy variety across the game; but by exercising that variety, an enormous additional cost shows up. Further, getting to a fight and seeing what is necessary should only require "reloading your gun" with the type of ammo most useful for the fight; it shouldn't risk making someone have to run back and "buy new ammo" in the form of additional inhalers now needed just because swapping out fireball and mirror image for lightning bolt and electro trap to deal with robots caused the player to completely lose their ability to cast psi. If you don't know ahead of time after the fight you just did what your next fight is going to be, then you're getting punished for having the "wrong" abilities readied. That's just a terrible thing to do to players exploring the game.
--Psychosis needs a way to recover the additional psi cost the feat causes. Now that psi is a globally scarce resource, Psychosis pays more "ammo cost" to use the same abilities, meaning once again, Psychosis has taken a step down in desirability. Making non-damaging and crit-incapable spells immune to the higher cost would be an elegant way to handle it but might be difficult on the coding side; adding in a psi reserve leeching system would probably be far simpler and might even feel more rewarding since it would be visible to the player. For example, letting crits refund some portion of the psi cost of the ability would be extremely in theme with Psychosis and would reward the fundamental play style; maybe 4% of the ability cost per target critically hit, capped at 20%, since the classic Psychosis opener is the AoE Cryo Orb; this then would also give a visible refund in exchange for the health cost of using Psionic Mania, making it feel more valuable. Psychosis just needs a little love to deal with this giant change more gracefully.
--Inefficiencies need to be dealt with similarly to other combat styles, rather than harsher. If you use a lot of "psi points" (bullets) in an SMG build, then your recourse is to bring more bullets because you need a lot. With inhalers only being usable outside combat, there is no mechanic currently to allow someone who has blown all their points on ineffective abilities to recover and keep fighting*. It's literally game over, reload. A third booster type that's usable in combat and restores psi reserve needs to be implemented so that psi can choose to "carry more bullets" into the fight if they need. Making said new booster scarce and expensive will ensure that it doesn't break the global scarcity of New Psi, while also allowing players who aren't doing everything perfectly to recover from their mistakes without getting punished too harshly for making them.
There are several others, but these three things are IMO the most important changes to be made immediately if full psi is meant to be an actual play style in UnderRail.
edit: * someone is going to say "but psi boosters can recover 50 psi reserve!" but that's not comparable at all. With a cooldown on when they can be used, and with 50 points only being 3 basic attack spells at 100% cost, a psi booster isn't sufficient. Guns have reloading belts; psi needs something to let them use as many "bullets" as necessary to finish the fight. The global scarcity comes between fights, not during them; you worry about doing merchant rounds to restock, not about rationing what's in your inventory when the shooting starts.