The unlimited breath weapons seems a little excessive, basically unlimited burning hands/aganazzar's with your chosen damage type. Not sure how to balance it, maybe CHA mod uses/ short rest, to retain its constant availability without it replacing a cantrip

The dragon forms seem way OP imo. Not sure how to balance it, maybe young dragons instead of adults?

Sleep breath is insanely good, sleep on failed save is ok, but stun(!) on a successful one is excessive. Also, hit dice is an outdated term that doesnt really have much meaning in 5e. Try CR perhaps, and removing the stun.

The ability to use more than one seems both OP and wasteful at the same time, given how few slots warlocks get. A multiclass warlock/anything else with slots could use these effects far more often.