Most of those things are kind of a matter of taste though aren't they? If you change those kinds of things you'll get other people complaining who like it as it is now. For example for me I think the default UI is excellent and the alternative ones I've tried are mostly terrible, but I know not everyone thinks the same way.
Other complaints are instance-specific but that's a good thing; instances can operate how they like because we have a choice, that's the whole benefit to Lemmy and federation.
Just to add to the other replies you've got, as far as I'm aware there's no reason why you can't add Fedora's flatpak repo on another distro. Why you would want to is another matter, but I think the fact that anyone can make their own repo is the fundamental strength of flatpak as opposed to snaps; it's not tied to one organisation, Flathub is the de facto central repo but it doesn't always have to be.