Well a few things. One it's abilities would be secret. Players are not supposed to look at a Monsters Statblock. Two the Terrasque has Magic Resistance and Legendary Resistance. Meaning it's very likely to save against anything you suggested and even if it does fail it can just use Legendary Resistance to negate that and save anyway. Also the Sorcerer with distant spell can you give me an example of a spell that would be useful that has a range of 600 ft and can be boosted to 1200?
Let me be clear. I am not saying that your version of the Tarrasque is bad. I'm not saying that it wouldn't present a challenge or that it doesn't deserve to be a CR 30 monster. Your version both restores some classic flavor and makes the monster significantly more challenging, and if you were my DM and I heard that the Tarrasque was waking up, and I cast Contact Other Plane or otherwise did some research and found out that the abilities you described were its abilities, my PC would be like, "Well, fooey" while I was a player would be going, "Yay!"
When I say that it still has vulnerabilities, I'm coming from the perspective of a PC who either:
1.) Doesn't know Wish and still wants to save his civilization from the Tarrasque;
2.) Knows Wish but doesn't want to risk burning it out;
3.) Wants to deal with the problem permanently, instead of risking it waking up again after the Wish in another few decades.
It is from this in-character viewpoint that I suggest things like a Sharpshooter kiting it into a Symbol of Insanity trap followed by Intellect Devouring and a ton of Shadows who drain all of its strength, then drowning it.
Bestow Curse can't permastun. (It can make it so that their is a chance each round that a creature won't act but it can't perma stun it.) and it requires the Tarrasque to fail it's save with Advantage and Legendary Resistance. Contagion requires the Terrasuqe to fail 3 saves before it takes effect. A shadow requires a Natural 20 to hit the Tarrasque. THe intellect Devourer's save is super easy for the Terrasque to make.
RE: Bestow Curse, you obviously know what I meant, so good. Yes, the Intellect Devourer perma-stun is more of a genuine perma-stun, and is much worse for the Tarrasque. The Tarrasque has a 91% chance to beat the Intellect Devourer's devour attempt each round, or an 82% chance if it is under the effect of Bane or if there's a Wild Sorcerer Bending Luck, or 69% if both Bane and Bend Luck are in play. (If you landed a Bestow Curse (Intelligence) as well its odds of making the save drop even further to 45%.) But all of this is overkill really. Even at 91%, a few Intellect Devourers will make short work of the beast once you have it disabled in an Symbol of Insanity (cast 70' off the ground, so that it will affect the Tarrasque but won't affect your Shadows/Intellect Devourers/etc. because they're not tall enough). The exact details are unimportant.
Against a civilization-destroying threat (or at least one with a civilization-destroying reputation), I
am going to engage in overkill, so in addition to the Intellect Devourers (getting those is probably an adventure in itself) I-as-a-PC would also be hitting it with everything else I can think of simultaneously, from shadows to Dominate Monster and Planar Bound Goristros and True Polymorphed dragons, just in case my information is inaccurate and the Tarrasque turns out to be immune to intellect devouring. But since I, as a forum reader, know that Intellect Devouring will actually work just fine, I can probably just stop writing here instead of detailing all of my paranoid plans.
Anyway even if reduced to 0 hp the Terrasque will keep on fighting until you use the Wish on it due to the fact it will just keep regenerating. If the Sharpshooter on Phantom Steed is annoying then the Tarrasque can just ignore it. I don't see how Pladorc and his Simulacrum will be able to overwhelm the thing in Melee.
Anyway it's still supposed to be beatable. And your entire thing pretty much shows a bunch of 20th level heroes that know all of the things weaknesses and strengths. Rather then a normal group of Adventurers that would run away. If this thing attacked a city or an army, Both the city and army would be crushed. (I always use the cleaving rules in my games so the Tarrasque would be wiping out 10 or so soliders with every swing.)
The key question for my enjoyment wouldn't be, "How are we going to beat this thing?" It would be, "How in the world did that thing, that we just killed, ever manage to take down the Ancient Empire of LostMagic?" If the DM has a good answer for that then I would enjoy the scenario. As I mentioned previously, the answer could be "LostMagic made some mistakes and didn't have all the information about the thing you did," or "LostMagic didn't catch the thing before it bifurcated ten times into a thousand Tarrasques and everybody, and then the Tarrasques ate each other," or "LostMagic was divided by internal politics, and they thought they knew how to deal with this thing but then LordBenedictArnold backstabbed Lord Mordenkainen right as they were about to execute their plan and the whole thing fell to pieces while Lord ShadyMcPuppeteer laughed his head off and then was awarded his own domain in Ravenloft to celebrate the tragedy."
If your players don't care about backstory and simulationism you probably don't even need to come up with that much.
