All that sounds great.

I personally like a good tank Tarrasque. The encounter is more puzzle than combat. The players are hardly noticed until they deal something like 10% of its HP. With a solid regen, that's a significant scratch.

Of course, this version would be boring in a vacuum, so there's a time pressure of it destroying a city.

This naturally gives it three stages. Stage 1, players are basically safe. They can try strategies, do shenanigans, and the Tarrasque will be at worst impeded, but likely relatively unaffected. The goal is just to get its attention at all. Any turns it spends targeting the players are turns the city is safe. Maybe give the city a magical artillery piece that needs to be set up.

Stage 2, they've delayed the city's destruction. Now the problem is the Tarrasque is looking their way. Their main goal shifts to some form of "survive". A clever party can lure the Tarrasque out of the city, perhaps into a trap if they had time. A lot of actions are spent maneuvering, escaping, setting up the trap if it exists. It's likely the Tarrasque fully heals during this stage, although that doesn't make it revert to stage 1. It may do so if it cannot get to them in any way, like a flyer 300' up.

Stage 3 is whenever it catches them or springs a trap. This is a final fight. Unless they straight up run away, this is when the big damage comes out, when that artillery may show up, when the trap springs. If they cannot do consistent damage, they will run out of resources and lose. If they can, they'll win, job well done, they get the keys to the city, a celebration, all that.