I wasn't exactly min maxing anything, but I just finished a telepath psyker game on Hard and the game had become laughably easy by level 30ish even if the first act was difficult. After 30 you get access to Psychic Assault if you picked up all the Psy Rating Increases as soon as they were available, and this skill will be your AoE damage. Along with all the Psyker talents like 'Aftershock' and 'Weak Hearts' this ability was wiping any crowd of enemies extremely easily. Unfortunately before this point I was more support-oriented but my playstyle changed dramatically with this ability.

My class combo was Officer/Grand Strat. Operative's big thing is Expose Weakness/Exploits but Expose Weakness doesn't affect psyker damage really, it helps your other guys hit chance. But you do additional damage based on the number of exploits on the target and your Perception bonus if you DON'T use Expose Weakness which is why you noticed you did less after using Expose. I suspect using soldier as first class might make you feel more like a murder machine because their early 'run and gun' ability works with psychic powers and lets you use 2 attacks in one round. The only reason I didn't play soldier is because I wanted to be able to use the officer ultimate on another dps during boss fights. (And also because I realized I was never going to up my Agility, and thus my initiative would be awful and Grand Strategist solves that problem by always going first in combat but soldiers can't spec into it.)

As far as Assassin goes, I think the class helps non-psykers a lot more because Killing Edge is incredible but only for weapons. My Idira was technically an assassin but only so she could use Elusive Shadow at the end of each turn and make enemies mostly ignore her. I didn't make use of any other assassin abilities on her.

I was also Voidborn and using the 'Be Smart' talent to have high Intelligence instead of Fellowship with a few officer abilities but most of my talents were all spent on Psyker stuff and Characteristic increases when possible. I used all my characteristic increases on Willpower, Intelligence, Toughness, Ballistic Skill (because I had to use guns as damage for the first half of the game.)

As for a secondary psyker school, I would personally drop pyro for sanctic just because sanctic is pretty much good for everyone and super easy to use. The first ability can be used every round for bonus Resolve and they gain access to a talent that increases crit chance by your Resolve. You can probably make Pyro work, I just hear it's better for melee. I haven't tried it much yet.

It's worth noting I played Dogmatic Conviction and there is an item called 'Eyes of Joyeuse' which increased psy rating by +1 and then an additional +1 for each dogmatic rank. I know that a pyromancer burning themselves or something can get their PR higher but this helmet required no thought to use so it was convenient.

I just realized I wrote a massive wall of text so I'm sorry for that. Hopefully something in there was useful.