Nobody has Earthbind.

I went into this thinking the dragon had the advantage so I didn't want to further stack things in the dragon's favor by having the starting range 100s of feet away. Since this was looked at as a gladiatorial contest, I figured 40ft was ok. I agree that the further the starting distance, the more it favors the dragon. I also assumed the party's tactics would be split up as much as possible. Even though the Rogue never did any damage on it's first turn, it was able to fly far away to the side away from the party. In the first combat, the Bard went far to the other side, and the Sorc went far back. In the second combat, neither had the opportunity to move away before the dragon got it's first turn, and both suffered from the breath weapon. I also assumed that any dragon would aim it's breath weapon at where ever it would hit the most amount of creatures.

In the first combat, it never got a chance to ignore the Barb and Fighter since it got grappled before it ever got a turn. My Barb has 20 Str, Expertise in Athletics, and of course advantage on Str checks so a huge advantage over the 23 Str dragon that's not proficient in Str saves/checks. My Barb never failed any Dex save at any time, but I'm not sure I would have allowed Wing Attack to cause the grapple to break anyway. It just says I'd have fallen prone, doesn't say I had to let go of my grapple. Also HIS movement is 0, not mine, I could have just stood up again while still holding the dragon in a headlock. How would you have ruled that just out of curiosity?

In the second combat, the Fighter hit it with 6 attacks on it's first and only turn so to the dragon, it was the most dangerous enemy and focused on taking the Fighter out which it did with it's breath weapon and 2 claw attacks one of which crit. It did largely ignore the Barb though having no attacks against it in the second combat. In the first combat ALL claw and tail attacks were against the Barb, but the Barb was too close for the bite attack or breath weapon range.

The Rogue proved to be an interesting foil for the dragon. With a +9 in initiative, it went before the dragon and was able to get away from the party and in position. Since it did no damage in either of it's first turns, I assumed the dragon didn't perceive it as a major threat at first. In the first combat, the dragon never had a choice, but in the second, it did in round 2. The Rogue has Lucky and Mobile so it could match the dragon's speed of 80ft not counting extra half movement from wing attack. With Bless, Lucky, Ring of Protection, +9 Dex Saves, and Expertise in Acrobatics, it was just never going to fail any Dex save and Uncanny Dodge and Evasion only helped to further mitigate damage as well taking no damage from the breath weapon. When it couldn't reach the dragon with Booming Blade, it hit with it's light crossbow. I think the Rogue would have been able to last at least one more round with the dragon before falling to 0HP. By the 3rd round, the dragon was strictly focused on the Rogue. The Rogue was hitting it HARD with Booming Blade with the dragon taking the extra damage each round and the +9 from the Aasimar racial ability helped a lot too. You could argue that the Rogue wouldn't be able to keep Bless up for the entire combat while taking so many hits. My group plays concentration checks a little more relaxed than RAW only rolling them on significant damage occurrences. I think I rolled the check on about half the hits and passed every time.

The Sorcerer has Spell Sniper so could hit it with Firebolt from far away, and got it to use it's Legendary Resistance twice with Fireball while still dealing like 15 damage to it each time.

With the Bard I realize I did make some mistakes. I was giving out Inspiration dice each turn, but was not in range most of the time, and also out of range for Vicious Mockery to work too. Not exactly groundbreaking, but still.

The 16 Con Bard and 18 Con Sorc both have 74/75 HP. The breath weapon was brutal knocking the Bard out with one hit and one miss for half damage. Luckily the Sorc was out of range after the first hit, or would have also been knocked out. The Barb could have tanked another hit or two. The breath weapon was a nonfactor for the Rogue.