I like how they've incorporated very individualistic elements to dragons, while still keeping the shared idea of a dragon as well as some of the traditional sub-race elements.
It's also a nice point about average or even low Dexterity scores as these are enormous creatures. The score is in relation to human average, so bigger creatures aren't typically going to be as deft as smaller ones - this time meaning the PCs. Plus, we get rid of the all-18s monster where they have no obvious weaknesses.
The idea of a single, permanent lair doesn't sound right to me. I think dragons do switch lair locations, if always desiring to have one (and the corresponding horde). What is the reason for Smaug abandoning his lair and taking up residence in the Lonely Mountain otherwise?
It would also make sense if dragons laid, hatched, and reared their young in their lairs. That these are probably remote and inhospitable to civilized lands probably follows as a truism. (Though I could see a sneaky shadow dragon rearing young under a human city perhaps)
Strangely enough, I'm going to suggest dragons aren't immune to their own breath weapons. They can be harmed if they claw or bite themselves. They can hurt themselves if they whack their heads with their tails. And we humans are susceptible to harm from our own physical abilities too. I think a basic Resistance to Breath Weapon attacks makes sense, but not full on immunity.
The weakest dragons in the very young category showed up in the Encounter tables for Monster Level 3 & 4. So yeah, Monster Level 5 probably is a good place to start for a standard dragon encounters.
Black dragons might not be amphibious, but simply hold their breath very well. This could mean literally hours between submersions though. Underwater and Waterborne encounters included Dragon Turtles, but no "true" dragons that I know of.
Heresy, I know, but maybe we could rearrange the order of dragon power / XP rating and base it upon ferocity of habitat like terrain, climate, and other monsters. For examples, polar bears are bigger than grizzlies and black bears are even smaller. Kodiaks are pretty big, but live in an isolated environment allowing that to happen. What if we changed the order of dragons just slightly, so swamp and jungle dragons were tougher, probably too arctic cold dragons, and forest dragons were weaker. Mountain peak dwelling Reds could still be "king of the hill", but they are hard to bump into for low level parties. Also, changing blue dragons habitats to shoreline territories would be just fine, but electrical discharge underwater is a death wish for both parties. I see them still as primarily flyers.