For me, there's a couple things here that have been issues in terms of finding the info I need & mental strain/fatigue... it's easy to say "oh I can handle that monster no problem" when looking at it in isolation, but - at least for me - it's when placed in actual play where I'm tracking a bunch of other things that it becomes burdensome. Anyhow, some things I notice...
Lots to Track: It's not just HP, conditions, and spells... but also Legendary Resistances, Recharge, and # Reactions used. This isn't different from 2014 legendary monsters, but it is a lot for the GM to keep in mind. IME unless I'm really "on" that night, the more my mental energy goes into tracking this stuff, the less mental capacity I have for great narrative and the rest of the game. It's not a zero-sum game, but there IS an influence - at least for me.
"Dead Space" Traits: Do I really need a dictionary definition of Amphibious or could that be a tag added in [brackets] after the monster type or its swim speed? Similarly, Battle Ready feels like it should be in the Initiative line rather than pulled apart as a one-off trait that I'm not going to look at for the rest of the combat, yet still takes up space.
Legendary Resistance: I'm really not sure what the extra use in Lair is adding – assuming Lair Actions are still in the game, those are plenty of reason for a party to want to lure a dragon out of its lair / avoid it in its lair.
Multiattack: This could be greatly tightened up, along with Rend, for example...
Multiattack. Rend x3, and can replace one Rend with casting an at-will spell.
Rend (15 ft) +15 hit, 17 (2d8+8) piercing and 10 (3d6) poison.
Spellcasting. There's already good reason to provide mini-descriptions of spells in the stat block... though this is more for people using physical medium rather than DNDBeyond where you can have hyperlinks – so I get why WotC is not interested in "spelling out" spells (sorry pun). However, when including upcast spells in a monster's stats, that creates an extra bit of reading the GM needs to track down to determine number of targets. Fine for a GM who has no trouble remember that Charm Monster is 4th level, and upcast at 5th gives it two targets instead of 1... but... not every GM is going to be there even when running high level. For me it's a question of WHICH spells get brief descriptions – and my go-to there is spells which have immediate combat application... Charm Monster, Dissonant Whispers, and Cloudkill. The others are more narrative spells used outside of combat (usually). These descriptions would need to be super brief in the 1-3 lines range.
Poison Breath: It's interesting how we insist on writing out "saving throw" rather than the colloquial "save" in stat blocks... But even keeping that, I think this could be tightened...
Poison Breath (re 5-6): DC 22 Constitution saving throw, 90-foot cone. Failure: 77 (22d6) poison. Success: half damage.
Reactions: "Introduction" to legendary reactions language can be tightened to two lines, and I think the "Trigger:" doesn't really save words compared to "When" and in some cases adds words without adding clarity, e.g. "Trigger: Another creature..." vs. "When a creature..." Also I can see tightening each of these, for example there is tons of space wasted on "a creature the dragon can see" type language throughout the monster books, so here's an idea...
Charming Presence. When a creature ends its turn, the dragon casts Charm Monster on just that creature.
Since "a creature the dragon can see" is baked into Charm Monster, no need to repeat that here.
Corrosive Miasma: When the dragon uses Legendary Resistance or is hit with a ranged attack, it spews a virulent orb at a point it can see within 90 feet. The orb explodes in a 30-foot-radius sphere of gas; DC 21 Constitution save. Failure: 14 (4d6) poison and -2 AC until end of target’s next turn.
This is only 57 words compared to the 63 of the stat block above, but it isn't only a little more succinct, but it's also clarifying the narrative of what's happening. Clarifying that's it's a poison GAS also more readily cues the GM in that strong winds might disperse it or magic items making one immune to gases would apply. And it's doing that with fewer words.
Rend Retaliation: When the dragon takes damage, it makes a Rend attack.
Less words, same idea.