Thanks for the work you have invested in using Flatpaks, but it is currently not usable for me.
This is my second attempt, where I try to get it to work on my XperiaX.
Perhaps the download and installation script can be optimized so that it also ensures the completeness of the downloaded data.
The downloads never seems to be complete (see screenshots). The installation of Angelfish always ends in a reboot. It says "Installation complete." displayed even though the download is incomplete.
I've never been lucky enough to have an icon created for Angelfish. However, starting from the terminal is possible and it appears to be working.
If you want to reinstall it because of the missing icon, the uninstallation of Angelfish ends 5-8 times with a reboot until it is finally installed and you can reinstall it.
Is there actually a force or override option for installing an app? That would be very helpful in such cases ;-).

Maybe I'm doing something wrong, I don't know.
But I have a bad feeling when the sizes of the downloads do not match.