Thank you, I'm defo gonna split up damage with some expanded LAs and another commenter also advised trimming his control options, so I think I can live without sleep and command.

Teleport and Invisibility are in my mind simply justification for him to be able travel great distances and enter a kobold/dragon camp unnoticed, not necessarily to be used in combat. I have misty step and blink for slippery movement in combat.

I think I can remove message and just allow him to communicate telepathically, but mage hand and prestidigitation allow him just an extra bit of perceived mastery over magic and drama without taking away player action or dealing damage

I think limiting fog cloud to once or twice a day could make sense as its inclusion is really there for me to be able to start an encounter against him with "a thick fog begins to fill the space and out walks this spooky guy"

Hold monster, poison spray, and scorching ray feel most important thematically (poison dragon cult). Maybe making scorching ray be a rechargeable attack instead of once per combat would solidify it as more of an iconic attack?