The problem is that you're combining a creature known for being solitary and long-lived with the ability to cast 9th level spells, which are especially overpowered when given prep time.

If the dragons are even the slightest bit intelligent, their lairs will be peppered with Glyphs of Warding, tons of True Polymorphed golems, and a series of Simulacrums each copied from the last (thanks to Nystul's Magic Aura letting it count as humanoid). And since the spells don't involve spell slots, those Simulacrums can cast their own 9th level spells. If you somehow do manage to kill the dragon, it will just revive from one of the many Clones in the Demiplanes it set up.