alright taking my look at this bad boi. So the 2024 MM designs really tried to highlight the "less is more" concept. I see Dave has already streamlined a lot of things in this statblock, and I will continue to ask that question. What in the block is really adding meat to this dragon, versus things that seem kind of fun but if were removed really don't change the encounter.
Tail Slam / Tail Sweep: At the end of the day, I don't think these bring that much to the table. For the slam, the fire blast brings you more range than the 60 feet reach and has a superior range. So you already have a good melee option in the bite and rend, and then the fire blast is an excellent long range option. I think the tail slam is superfulous. On the sweep, the Wing attack and the sweep really do a lot of the same things (wide area attack that does prone, and allows the dragon to distance themselves from a target with a high dex saving throw). Of the two, I think the wing attack is really all you need, great flexible AOE that gives you a prone and blind attack and then lets the dragon reposition as it needs to. The tail sweep has some niche moments, but if it was gone honestly wouldn't miss it a bit, the wing attack provides everything you need.
DCs: So we have DC 24s, 25s, and 26s in this statblock. One thing as a Dm, I HATE multiple spell DCs. hard for me to remember and a night for my players to remember (hell even if a creature has a single spell DC I can't tell you how often I have to repeat the DC on every save....with multiple DCs there isn't a hope and a prayer my players will remember). I would just pick a lane and go with it.
Noxious Smoke: Language needs some cleanup. What this effect does is simply creature an emanation that lasts 1 minute. Then if someone starts their turn in it they take the penalty. the language jumples these two things together and its a bit confusing.
Anti-Magic Field: So one rules question. Because the dragon attacks are "magical slashing damage"....do they work in an anti-magic field? I am sure the answer is supposed to be yes, but its a reasonable question for someone to ask.
Legendary Actions: So 2024 has made a concerted effort to get away from the "2 action/3 action" type legendary actions. So I would ask if we can push that here? On the fire breath, I see the plan but I agree that its an odd design, goes against the new 2024 aesthetic, and also puts a lot of pressure on the very next pc after the charged breath, as they basically have to come up with some magic to stop the party from getting fried. So let me suggest a possible alternative:
Charge Breath: Now a legendary action, no longer a recharge rate (I liked your old dragon models where we got rid of the random chance much better)
Fire Breath: Now an action and as the clause (when used, the dragon cannot take any legendary actions until the start of its next turn). Would also love to get that damage into the 200ish range, because at that point you actually threaten instant death (aka double max hp) against some of the lower hp characters. If anything should be threatened the insta kill at CR27, its a dragon's breath weapon to me.
So that mostly mirrors what you are going for, but in a cleaner 2024 aesthetic (its not a perfect replication but I think it gets to the heart of what you are going for.
In terms of how this dragon plays, the fact that fire breath cannot be stopped by resistance/immunity is a big factor. It can do a lot of damage and to me is a good round 2 kind of threat, if you have weakened a lot of players in round 1 it can be a great finisher.
Anti-magic field is the ultimate trump against many high level shenanagans. It allows the dragon to negate things like walls of force to corral it or getting tripped up by a lot of high level magical users. I think it would be a go to move for the dragon against many high level parties, and probably a "wish, negate a dragon's anti magic field" would be a classic move expected to go toe to toe with this beast.
The wall of fire can be surprisingly effective in the right terrain for its ability to block line of sight (though the dragon's height is a weakness here, it would need to be in places like tunnels where it can duck being the wall to truly prevent sighting. For the power word kill, what is great about this in the arsenal is that so many high level PCs have a "at 0 hp you get x, y, z" so an ability that just takes them out is great. Its also a good true finisher, if the dragon knocks a pc down to 0 with its first attack, it can truly finish the job with power word kill. while that might seem like overkill, with lvl 20 pcs you can never be too definitive in taking them out of the fight, and this one way to truely do the job.
I think the real power though in the spell list is the forcecage. Simply put, this is a nigh guaranteed way to take a PC out of the fight every round. Even if they have teleportation, they still need to make a charisma saving throw (and against that dragons' DCs that is not happening for a lot of PCs). A smart dragon could easily knock out the rogue or like a barb for the fight.
So with that in mind, I would question the forcecage in the arsenal. It is clearly extremely useful...but I feel like it dominates the statblock a bit too much. The dragon has a number of other good conditions that are strong but the players can overcome them or just suffer them 1 round. The forcecage can just shut down a PC after pc for the entire fight with often few ways to get out of them, and turns this from a fight about the party beating an engine of destruction to them fighting a high level arch mage.