I´m not going to try to convince you to change anything about your one shot, far from it, but maybe my experience could be useful, since I have ran a kinda simmilar encounter. The BBEG of my last and first campaing was an adult red shadow dragon, let´s name him Xiu. My players were 5 level 15 with a lot of powerful magic items, ranger, rogue, artificer, paladin and druid.
I buff dragons a lot, with a freebie attack as a reaction, a free breath attack at half health, an elemental aura, stronger spellcasting, new actions and legendary actions, you name it. He was also in possesion of the Ring of Winter, a very powerful artifact from ToA, and at the end of a very hard dungeon; the caldera of an active volcano, lava everywere, extreme heat dealing damage an exhaustion, toxic gas inflicting poison, efreeti, iron golems constantly healing because of the flames, githyanki and abishai squads, undead dragons, two different homebrew CR 19 monsters immune to fire and an arcanaloth with a bunch of yuan-ti, all of them serving the dragon with only one short rest halfway in. I also played the dragon like a true bastard, never landing and swimming in the lava, cornering the party in a small walkway were all of the party but the ranger were forced to take the breath weapon, killing the healer first, grabbing the PCs and sinking them into the lava, taking 18d10 fire damage... The dragon by himself, not taking the lava or the artifact into account was CR 22 or CR 23 with all of the buffs. He was a solo monster, but the part was absolutely spent.
They won, and the only one that died was the paladin, who was revived in the epilogue. They won by the skin of their teeth, but no one else dropped to 0. High level players can deal with things like that. My players defeated the dragon, so i wouldn´t be surprised if your players defeat your dragon easily.
On a related note, what other poster said, you can get crazy with the encounters at these levels, it´s great. We started another campaing, and while I gratly enjoy it, can´t wait to reach the high levels again.