A limitation of distro agnostic software centers like discover and gnome-software is they cannot just read the metadata every package manager already has for packages in the repo it must have its own xml data as well as mandating icons and a .desktop file... Which most packages do not ship by default requiring maintainers to add them for every package to appear.
So there is a lot more in the repo than the software centers will ever show. The alternative to this is using eopkg directly such as sudo eopkg it fastfetch or installing and using solseek which is a TUI (Text User Interface) made by a community member who recently became Solus staff which calls eopkg, flatpak etc for you while proving a nice interface.
This is what solseek looks like:
