You can keep your purchases these days.

Though I when last changed countries I did find that I lost some purchases. I would guess it can happen when an item is not available in the new country.

There’s a rate limit on how often you change countries and you’ll need a payment method from the country you want to change to (e.g. a credit card issued in that country, the address on the card doesn’t matter).

You’ll also need to cancel e.g. your Apple TV+ subscription before you can switch.

No, not for the App Store account — which can be a different one. Once you've downloaded an app with a different account, the credentials will be stored and you can switch back to the native one; you'll also get regular updates. I do this all the time for US-only apps on an EU phone (e.g. Google Voice).

I'm pretty sure I changed regions once on my old iPhone over a decade ago and it was fairly seamless.

Maybe someone who's done it more recently can comment?

It would be brutal if what you say is true. Simply moving is not something a user would expect to reset their whole account.

And if purchases were lost I would think it would be pretty strong grounds to bring a lawsuit against the company (especially if the apps were bought prior to any attempts by Apple to write in clauses permitting what'd amount to theft).