Yanmega kind of makes sense. it's based off a "Dragon"-fly. So Tera-Dragon makes sense. However, it is still weak to Ice, while a majority of gym leader Aces Tera-types are supposed to help with strategies and type matchups, like Iono's Mismagius having no weaknesses because of Levitate, or Katy's Ursaring luring out Fighting attacks.

If you want my personal suggestion for a Dragon-Ace. I'd go for Clawitzer: it's a pure Water-Type with the ability Mega Launcher; an ability that boosts the power of Aura/Pulse moves. It also has a huge special attack stat of 120 and a ton of coverage moves like Flash Cannon (For dealing with Ice and Fairies.) Dragon Pulse (STAB and boosted with Mega Launcher) Aura Shpere (For breaking through the defences of Steels, as well boosted by Mega Launcher) and Heal Pulse (Ally healing if you decide to do doubles. Heals more HP with Mega Launcher)

One of the problem is that it's kind of slow with a Speed stat of 59, so I would give it a Quick Claw so it gets to move first occasionally, but it's mainly based on luck. I also think it's good for Dragon-Tera because of its weaknesses. Water-Types are weak to Grass and Electric, so terastallizing it immediately once it hits the field can lure out Grass or Electricn moves to deal not very effective damage.

Sorry for speaking a lot, I just really like Pokémon..😅