3.5 was edition I played the most. It was a reason why I quit RPGs for nearly a decade because I hated it so much.
Every time I see another meme about how amazing 3.5 Tarrasque is, I remmember how amogn actual 3.5 players Tarrasque was the biggest joke. It was always brought up as definite proof designers have no idea how to make good monster. It was laughably easy to beat. A wizard could casually solo it, the same abilities people now miss in 3.5 amounted to ribbons. It was a laughingstock, forums had 100+ pages discussions how to fix it and general consensus was it';s beyond saving. It was first proof in 3.5 if you cannot use magic you're only good to roll over and die.
I honestly don't know if everyone claiming 3.5 Tarrasque is such a horrifying monster are trying to rewrite history or unintentionally proving what a broken, unplayable pile of garbage 3.5 was, if it's biggest punching bag is actually dangerous in a different, better designed game.
Giving the Tarrasque a burrow speed goes a long way towards improving it, I agree. The hurl boulder ability of giants wouldn't hurt either, although there's really nothing stopping a Tarrasque from hurling rocks with a standard BAB.
I wouldn't mind giving the Tarrasque a breath weapon, either. It works for Godzilla.
But these are incidental improvements. Just ambushing players in a cave will go a long way towards negating it's deficiencies, even at high levels.
Catapults are popular for a reason. But there's still some issue of ammo and opportunity. You're really banking on your target just hanging out at the optimal firing range.