If that is your ideal setup, then I think VanillaOS and its apx package manager might be of interest to you.

The main benefit of Flatpaks for me as a user, is that I can upgrade my system without fear of anything breaking (I use Silverblue, which relies heavily on Flatpak to enable this).

I think you should look at the runtimes basically as a repository. There are a bunch of libraries in there, and you make sure that your application works with those versions. Except that now, these libraries and versions are consistent across distributions, so you can support multiple distributions in one go. Additionally, it’s the application developer, who knows the application well, who ensures this compatibility, rather than a packager. Which, again, benefits me as a user, in that I can use the app even if my distro doesn’t have someone to package it.