I got my Psyker to lvl 30, and the perk that sacrifices head-pop stacks to set enemies on fire with the push is a real game-changer. I never realized before just how far the push reaches. Most of the small-to-medium rooms in most maps, I can walk into the doorway, unleash my push, and watch literally every enemy in the room ignite; then I duck back behind the doorway and wait for the survivors to come after me so I can chop them down as they round the corner.
Combine this with the perk that gives free head-pop stacks whenever nearby allies get kills, and now I'm unleashing the push pretty much off-cooldown for lots of easy damage, where before lvl 30 I rarely used it at all.
Last week after I finished all my weekly contracts on my Psyker I started playing a Zealot. Coming back to my Psyker this week, I realize that there are a lot of game mechanics I did not really appreciate before playing the Zealot. Like how ranged enemies stop shooting when a player gets close, so it is often better to take a few steps forward and hack away at enemies rather than trying to dodge and shoot them at medium range. And how these enemies are easier to deal with if I don't alert them with a head-pop from across the map, instead letting my Zealot allies sneak up on them first.
I tried the Flamethrower staff, and I don't love it. Compared to Voidstrike, Flamethrower has a wider cone and unlimited pierce, and applies a damage-over-time effect. Flamethrower has a much shorter range however, and takes much longer to kill enemies. I think Voidstrike's ability to instantly kill enemies at any range is the more valuable option.