But your basically saying the god of Fate and magic shouldn't have stronger psykers.
Nurgle and Slaanesh can have psykers iv played them before, it's very difficult I admit, but as long as you run unaligned for the talents, then when u get what u want drop extra xp into the god u actually want to worship. Nurgle is incredibly easy with the sound constitution. It's tier 1 and can be purchased multiple times, it wI'll cover anything u get from Tzeentch unless ur just not building correctly. It's going to cost more I agree but that doesn't make u any less of a psyker.
What we did was had a preprerequisite of 50 cp and a mark, from there we had a minor campaign that u would dedicate to a god to get his attention and ask for the gift of psyhood. For Slaanesh, filling urself and as many people u can with excess for a 6 weeks, for Nurgle the spreading of a plague on a hive world effectively turning them into plague zombies overy the course of 7 weeks. And then we gave Tzeentch's mark players a +2 to the already +1 so, +3 psy rating so they still feel like their devotion was worth it. These campaigns took a session each, and had other goals behind it for other players. The one who wanted the powers had to convince his warband to help him aND even had offered them favors. Everyone was happy.
Edited by DarthKaleashth