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.