Do you mind a Rolling Release? If not, try Arch, either Vanilla with the AUR enabled or EndeavourOS. The default package manager (Pacman) + the AUR has pretty much everything you could need without resorting to flatpaks, snaps, or appimages (or, did you mean something else when you said you’d “like to use the default package manager exclusively”?).
If you want something you don’t have to constantly babysit, OpenSUSE Leap is a good choice and has as big of a selection as Arch does, or so I heard, anyways. There’s also Ubuntu if you don’t mind Canonical’s Snaps (or know how to get rid of em). You also get the benefits of DEB packages IIRC, but don’t know if you’d count them as part of the “Default Package Manager” or not…and off the top of my head, that’s it. I’m sure there’s more, but none that you can get away with only using native files on it are coming to me