I wanted to point out that there have been some significant efforts to address this with the chum work to get QT 5.15 and a significant number of kde libs and apps running via the obs at build.sailfish.org.
That work could be built on to get much more modern packages into chum. The main work requires a bit of study of the way @rinigus and @piggz setup the builds, but even an rpm packaging noob like me was able to make some contributions. Developer Announcement - Qt 5.15 available for app developer testing
Obviously, this is primarily aimed at kde apps, but can be used to support a lot of apps. It’s not just ‘fire and go’ installs, but editing some spec files is certainly a lot less work than building by hand.
That does not detract from the idea of having nixpkgs running on sfos. I’d just like to point out that systematic efforts to have automated builds of more modern apps has been don.