I think this is your only option, aside from waiting for the gnome-3-38 extension.
Releasing gnome-3-28 for core20 isn’t so simple because the extension relies on a content snap to do all the hard work, and this content snap is only released for core18. Porting this to core20 would probably require the same amount of work as creating the gnome-3-38 extension.
You can take a look at what this person was doing to get an idea of what is required. They also need a newer version of glibc and so they have to do everything the extension does manually.
You can also take a look at the snapcraft desktop helpers project to see what we did before extensions and content snaps existed. You’ll need to replicate this in your snap.