I've played the Lost Mine of Phandelver beginner box campaign for 5e twice. Both times, I ran the dragon that appears in it by adapting this guide. It turns what would otherwise be a very dry and boring encounter in a very enclosed space into something that absolutely terrifies the players.
Having checked out that comment: Yes, that's absolutely the correct way to run your dragons! Making use of that intelligence of theirs, using every tool at their disposal, is the difference between a forgettable one-off piece of the day, and an encounter with the king or queen of fantasy itself, personified.
Of all monsters that you absolutely must not disrespect, either as a player when you encounter them or as a DM doing worldbuilding and running an encounter, it's the dragons. They're some of the only monsters that I will allow to go no-holds-barred against players, because even if a dragon does kill a player: If they do it with enough style, strategy and awe? Then that PC can live on in heroic infamy, possibly become a defining part of the party's story. The PCs see one and know it's time to bring their A-game, and if they win, they know that they seriously earned it. There's no way you can get that kind of impact by just slinging your dragon into melee with the fighter and watching its' HP tick down as the Action Economy takes effect.
First times I ran one, I made him land to be fair. Got whiped out. Was fun for the players.
Then another, after I Dmed a one shot of my creation, my players gave me a gift of their own. They let me use an Ancient red dragon against them at full force.
Final box (low Res I made this exclusively with the samsung gallery app). But not being silly, I had basically no dragon lore for my setting which was really really full of cunning strategists with powerful abilities. It made the most sense to simplify dragons into the top tier forces of nature with no spellcasting and near beastial minds, but still capable of destroying a town with that alone, because that's what my world was lacking.
Oh, you know what I think they might benefit from for you, then?
In past editions of DnD, aside from damage resistances, then it was also possible to get abilities that would reduce all damage taken by a flat amount. And this would be perfect for your dragons, because normally a key problem with 5th edition is that the deadliest encounter in the game is 50 town guards.
But, if you give your dragons the trait that they reduce all incoming damage by 10 points...
A) They can now just ungabunga charge into a town and destroy it without instantly getting slaughtered by the town guard as soon as they go into melee (Normally dragons in 5e get their asses handed to them in melee just because of the Action Economy and the fact AC at high levels doesn't function, it's why tactics are so important to a good dragon encounter). A 10 point reduction means even the mightiest blows from a guard are only going to deal 1 or 2 points of damage.
B) Flat reductions are also something much more easily overcome by players- ESPECIALLY players that have a magical high damage sword. While a resistance gets better the more damage you take from each attack, a flat reduction can quickly become irrelevant.
That reminds me of the damage thresholds on vehicles in 5e. Thanks for the idea it's a really cool way to make them feel like the forces of nature that they're supposed to be!