I really like this. Going to tweak it a bit though. Make the seventh level give you a Young Dragon mount of the colour of your Dragon Master (that doesn't take actions to command come 15th level), make it require knowing Draconic, and probably replace the capstone with something that feels more... Dragon-y.
has also inspired me to try and write up a Dragon Patron for the Warlock.
has also inspired me to try and write up a Dragon Patron for the Warlock.