I agree with everything you said, however there are a few details.

it is technically possible to just launch your own [repo]

The ability to create repositories ≠ from mirror existing ones.

Unlike apt repositories Flatpak ones aren't simply a directory tree with a bunch of files that can get mirrored using rysnc or other efficient means, it's a clusterfuck of HTTP-only requests that need to be backed by specific metadata and there aren't tools to manage those.

flatpak create-usb may be promising but the name says its all - the priority wasn't to create a way to mirror repositories but a quick and dirty hack for some situation.

theoretically you could open your own repo and throw all dependency related packages in there or am I getting something wrong here

Theoretically yes, in practice things are bit more nuanced. That tools only considers your current architecture, it's a pain to get dependencies in an automated way and most of the time you'll end up with broken archives. You'll also need to hack things a lot.