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.












They could, if they wanted to. This is somewhat an example of “if it isn’t broken, don’t fix it”.
A lot of that is the fact that Linux is run incredibly lean. Replicating that isn’t cheap. They absolutely can, but since Linux is free and they can even modify it to suit their needs, its far simpler to do that.
Android is the best example. Google wanted a phone OS, so they bought Android Inc, who was making one. They could’ve spun up their own with their own talent, hired more, etc, but just absorbed one instead. That talent was making a phone OS based on Linux, because, again, they could’ve delved into the details of OS creation, but it was far easier to take a free OS, change the bits you want changed (like adding touchscreen support, which to my knowledge, wasn’t in the Linux version Android started with), and run with the new version you’ve made.
It’s also worth pointing out that Google has spent a lot of money on Android, and other large companies spend a lot on developing their own custom Linux Distro. It’s not like they have one software engineer for Android who downloaded Linux once and changed it. These companies are willing to do what you describe, they just didn’t have to reinvent the wheel. The Linux kernel, thanks to the community behind it, is incredibly secure and efficient, and there just wasn’t any reason to change it or copy it when it exists and is free to use.