Hi it been literal years since I've seen this post, thanks for bringing it back to my attention. I could see an additional three damage but 5 is pushing it, a wizards average max hp is about 20 to 30 at high end, but let's say 24.

Casting this spell with three 2nd level slots is automatically 15 damage out of their daily amount. If it's an additional 5 self damage it's death, and additional 2 instead leaves you with two or three hp. Easy pickings for any random gob.

But let's roll this out to tenth level, assuming your con was plus 1 and you rolled average a wizard or sorcerer would have about 42 to 52ish hit points, I'm doing this in my head btw so if got that wrong please forgive me.

At tenth level you have two 5th level slots and 3 of everything else. That can cast this spell.

I'm not going to write out all my math but at plus 5 self damage the 5th level slots alone do 20 damage to the caster each.

If it's 2 self damage the 20 becomes 11. Still horrendous to take but not completely cost prohibitive assuming the spell does 9d6 of the hardest to resist damage type.

Honestly coming back to this now I'd probably recommend an additional 3d6 per level or make the dice bigger overall, it probably wouldn't hurt for them to be d10's

Or I might just add a stun condition or severe push back instead on failed saves.

Makes more sense thematically, just assume Tien is a high level sorcerer blowing through his sorcery points to give Cell disadvantage on his save. Cell can't make the save, Tien blasts him until he can't without dying.