@silverwizard
single enemies work badly in D&D and often lead to anticlimaxes
In what way? Too easy to defeat, even if by the book their threat rating is too high?

My idea for this fight (as it's in my head right now at least) is that it won't be simply be a matter of hitting each other and grinding down the hit points. At first, the dragon should be way too powerful for them, and they need to figure out how to even fight it. Once they figure that out, it shouldn't be too hard to kill it.

I think the dragon will have a ton of protective magic that will make it hard to hurt, so they will have to figure out a way around that. Or to dispel it. And for that, I'm thinking about an anti-magic orb, perhaps with a slightly larger radius than normal, that covers some of the tunnels into his lair that the dragon has trouble entering, so anyone entering through there will have all their magic deactivated. This will initially be a problem to the PCs, but one they find the orb, they can use it against the dragon.

use your legendary and lair actions to spice up the fight more than hurt
Yes! Look at the stats for 5e dragons, I was mostly disappointed because they're kinda boring compared to the Shadowrun dragons I'm used to, but those legendary and lair actions are cool, and I will definitely use them.

let them be 14 and do some Hit Thing With Axe, it's fun!
Considering the CR of the dragon, I'm definitely considering letting them be a bit higher level. But why level 14 specifically?

Also, once they've employed the anti-magic orb, I guess hitting it with their axe will be the only way to hurt it.

Of course they also still need a way to tie it to the ground, or it will just fly off. I've been thinking of making the lair a crater and dropping a big ship in there. To show off how big and strong the dragon is, but also to give them access to plenty of ropes and chains.

Listen to them make characters and see which subsystems they like, be familiar and make them have cool things hidden inside (someone likes hiding and sneakattacking, clump of stalagmites that they can hide in, someone likes fireball, group of like 10 kobolds doing a ritual that will become a big problem if not for a fireball)
I'm not entirely sure yet how they're going to make characters, but I think they'll make them in advance. Otherwise it will cut into play time. I do know they love abusing sneak attacks; one player plays a bugbear with ridiculous amounts of sneak attack damage in one of their campaigns.