This kind of vigilance is exhausting, and thats the point. Canonical knows that abandoning Ubuntu for Debian will come with a cost, and they're angling that the cost of abandoning Ubuntu is higher than the annoyance youll experience from the advertisements they've shoved in to Apt, or the fact that they partner with Amazon out of the box.
Eugen knows that migrating an instance (or even a single user) off mastodon and on to something else is a big, fragile task. (Hometown exists, and is significantly less GrowthHacky than mastodon, but is a direct migration and mastodon derivative. OTOH, as a mastodon derivative, most of the dev effort has to go in to either improving or undoing what's been done to mainline mastodon.
Account migration exists and works, even to other platforms, but you lose post history (which is a bigger deal for some people than others.)
I'm not saying it's impossible, it's just Effort.)