Re: interesting Until....
"If you care about packaging systems then this is not the distro for you."
I have to admit that my initial encounter with Flatpak long ago wasn't very encouraging. The framework thingy which was supposed to sit on top of the distro and provide the libraries or whatever for all the little Flat things to sit on, although allegedly the correct one, wouldn't install, whingeing that something or other it needed wasn't there and wasn't available to install. If the solution that's going to solve installation problems had installation problems it was obviously too broken to consider further so I haven't considered it further. Add to that the fact there were several of these schemes all competing to be the solution for installation problems and it looked as if the solutions were collectively more like a problem in themselves.
So it's not so much that I care about packaging systems, rather than caring not to use any such system above what Debian/Devuan provide out of the box.
AFAICS for something over and above the distro provided packages it's simply a matter of parking a directory in /opt* and that can be done by simply unzipping or untarring it there (e.g. Seamonkey) using the distro-native format but installing in /opt rather than /user (e.g. LibreOffice) or some sort of contraption of its own (e.g. Informix which has a Java installer). Anything more just complicates what should be simple.
* With an entry for the launcher in the desktop manager's menu if needed.