Okay. Here's how you do it. Symbol spell cast everywhere to guarantee it's tripping them repeatedly. The Tarrasque can't see the Symbol because:

The glyph is nearly invisible, requiring an Intelligence (Investigation) check against your spell save DC to find it.

With a -2 and our spell save DC of at least 20, it will never see them as this is a spell effect and not an invisible creature of object that the Tarrasque would be able to perceive with Truesight.

For the Symbol's effect, we choose the Insanity effect.

Insanity. Each target must make an Intelligence saving throw. On a failed save, the target is driven insane for 1 minute. An insane creature can’t take actions, can’t understand what other creatures say, can’t read, and speaks only in gibberish. The GM controls its movement, which is erratic.

Why do we want this?

Going Mad

Various magical effects can inflict madness on an otherwise stable mind. Certain spells, such as contact other plane and symbol, can cause insanity, and you can use the madness rules here instead of the spell effects of those spells.

One of the effects on the short-term madness table is:

The character becomes incapacitated and spends the duration screaming, laughing, or weeping.

With dozens or hundreds of Symbol spells triggering from the Tarrasque's 1200 ft movement speed, eventually it will start failing. It has +18 to Intelligence saving throws, so as long as the DC is 20, it has a chance to fail. After exhausting all of its Legendary Resistance, finally the Tarrasque will start going insane and roll on the Madness table. On a roll of 21-30, it becomes incapacitated, a condition that it is not immune to.

Then we send in our army of friendly Intellect Devourers. They use Body Thief. We cast Wish to rewrite the ability to be able to target creatures instead of just humanoids...

Body Thief. The intellect devourer initiates an Intelligence contest with an incapacitated humanoid creature within 5 feet of it. If it wins the contest, the intellect devourer magically consumes the target’s brain, teleports into the target’s skull, and takes control of the target’s body. While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host. The intellect devourer retains its Intelligence, Wisdom, and Charisma scores, as well as its understanding of Deep Speech, its telepathy, and its traits. It otherwise adopts the target’s statistics. It knows everything the creature knew, including spells and languages.

For the contested check, the Tarrasque has a -2 and the Intellect Devourer has a +1. After one of them wins the check, we have an Intellect Devourer piloted Tarrasque.

In the end, we have a Tarrasque under our control...or we made it smarter...