Sorry for the slow reply, life has been interesting recently...
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May not answer you question directly.
There's something called software center. Like discover[1] for KDE plasma,
gnome-software[2] for GNOME.
Users can install either debian package, or flatpak, or snap apps.
[1] https://tracker.debian.org/pkg/plasma-discover
[2] https://tracker.debian.org/pkg/gnome-software
Ah, yes. That sounds familiar. Maybe this should be the location to implement that functionality, and implement it upstream. Maybe such a functionality could be shared and eventually integrated into aptitude and even apt itself.
My primary concern is to prevent disruption to users. That's a philosophical issue far beyond the two packages of mine that are currently impacted. If these software centers were able to smartly suggest a different source for a software package in a situation where that package was removed from the source the user was currently using, that would certainly address my concerns. It would make life easier for me as well because then I would just outright remove my problem packages from Debian without fear of negatively impacting users. (I've been on the receiving end of that impact a couple times and it was unpleasant to suddenly realize that a system upgrade made me lose a piece of software that I routinely use.)
-Olek