The party is really optimized with higher than normal stats. 3 of the players are 11, 13, and 14, but they know their characters well and how to use them.

I feel like I'd I controlled an adult dragon of any color against my party I'd destroy them easily. As strong as our party is, we really aren't built for ranged combat.

My Barbarian/Sorcerer only has hand axes as his ranged attack.
The Battlemaster Str Fighter only has javelins.
The Swashbuckler and Bard have either hand or light crossbows.
The Pryo Sorcerer has NO weapons, and is loaded down with fire damage spells so all sorcery points will be spent on Transmute spell to do any damage at all.
The Bard has some good spells, but has to make it through the legendary resistance to land any of them.

The Bard, Sorcerer, and Swashbuckler have HP in the 70s and adult red dragon's breath weapon averages 63 on a hit. Yeah the Rogue and Sorcerer(and Barb) are basically resistant, but I think the Bard will go down quickly. That's why I think Greater Invisibility may be the best option for concentration. The Sorcerer can twin Haste. My Barb can cast Jump which may help(we are NOT following the no spells while raging rule BTW, but he can't control his spells very well either). The Bard can maybe Enlarge my Barb and I can try to grapple the dragon. I have 20Str, Advantage AND Expertise in Athletics, but adult red dragons have 27Str.

The potential 6th character would likely be optimized for fighting a dragon though.

We also have a retired OP Reborn/Revenant Tempest Cleric retooled for Cold and Necrotic damage instead of Lightning and Thunder with an all new spell list to acomodate. We'd probably win with this character playing.