Hot damn, now that's a scary breath weapon! Honestly it might even be too much, though of course at CR 30 its literally kitchen sink time for the DM. A possible alternative (though I still like the original model so I'm not sold this is better, just different).
Breath Weapon: If a target fails their saving throw by 10 or more, they are considered to have failed their save against a
disintegrate spell.
On the one hand, your method does a lot more raw damage. But a disintegrate does have the advantage of killing them at 0 and preventing any "auto recoveries" when you hit 0 that some creatures get, so it does have some spice to it. It also forces "real" resurrection, not that 5th level bargain bin stuff
Purely for flavor, but I thought about the idea that this dragon is so scary that it lingers for days or even weeks (I figure in the high CRs we can just go a bit ham and over the top). See what you think.
Frightful Presence. Each creature of the dragon’s choice within 240 feet of the dragon and aware of it must succeed on a DC 26 Wisdom saving throw or become frightened for 1 minute
and suffers from the Creature of Nightmares effect on their next long rest. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the dragon’s Frightful Presence for the next 24 hours.
Creature of Nightmares: The next time the target takes a long rest, it is subject to the nightmare version of the
Dream spell, with a saving throw equal to the frightful presence DC. The target is subject to this effect on each long rest until it passes the saving throw.
That is mostly just a cut and paste of the lesser goddess version of
