Yeah, 5e's flatter math is not friendly to single large beasts, like dragons. But I don't know if the math is what needs to be changed either. I think it's the action economy that's the problem. It seems that if a creature only gets 1 turn/round, then either it doesn't last on the field long enough to be interesting, doesn't do enough damage to be threatening, or can simply kill a player, which isn't fun either. So then the solution would be to have multiple creatures, but that isn't necessarily the best idea either.
Perhaps making a creature that can actually take multiple turns/round while still being one creature? This way its damage per turn can be toned down to something appropriate to the party, but have its overall threat level heightened enough that you can get by with only one.
(I don't know if Oberoni applies here. It's the idea that a problem doesn't exist because it can be houseruled, whereas what I'm trying to do is acknowledge there's a problem, and offering suggestions to how to go about it)