Just had a thought, what if you made it so that instead of using up all the psyker’s warp charges automatically when the player uses their ability, this function is activated by the player pressing their ability button while it’s on cooldown? Maybe it could be one at a time; press ability button to eat one warp charge and chop a few seconds off the remaining cooldown.
This would let players make the tactical decision to use or not use them up. It wouldn’t change the game at all for players who only have the main keystone talent; they can use all their warp charges as soon as they use their ability. It would also let psykers potentially spend more than 6 warp charges on reducing their ability’s cooldown if there are elites and specials around to kill to get more warp charges quickly, which might be a bit too powerful and need to be reigned in in some other way, but it would feel great since it would synergize well with all the psyker’s other elite killing skills.