Yeah, dragon spellcasting (which is absolutely NOT optional if you want to run great dragons) has three components:

-Their Spellcasting Ability is Charisma, and they also have a number of spellslots equal to their Charisma Modifier

-Their spell slots are of a level equal to 1/3rd or their Challenge Rating, rounded down. Like the warlock, all their slots are the same level (and I refresh them on a Short Rest, dragons are meant to be magical after all)

-They can learn spells from the Sorcerer spell list by default. Personally I use different spell lists for different dragons, but Perennias was a straight up sorcerer.

In any case she would have had to actively tone down the spell she'd use for a giga AoE to 7th level so that her Ring of Maximum Power could actually trigger, because yes, all her spells were otherwise 8th level by default.

And yes! You get it! You understand! The essence of a great magic item loadout is to have a strategy. Think about what the dragon wants to act like. At base, they're not as good at fighting as a Fighter and not as good at spellcasting as a Sorcerer, but their magic items can allow them to surpass their rival PCs and become truly terrifying.

Crown of the Vengeful Dominator is also a really sweet drop for Warlocks, since normally spells upcast incredibly inefficiently, making a warlock's spells from each previous level suck compared to the new spells they get. But with that crown, their old spells don't get outdated nearly so badly.