D&D 3.5 handled it by giving psionicists a bunch of psionic points that they could expend for "spells". They learned abilities at level up, rather than memorize them, so they were really more like sorcerers turned up to 11. The fact that they could dump all their points on their highest level abilities was balanced by the fact that they would have terrible endurance if they did that. They would be OP in an environment where they could rest between each battle...
Still, I wish they were in 5e. I love every book by Jack Vance that I've read (and I've read a bunch) but I hate the stranglehold he has on the D&D magic system.
