freebsd-update should be able to list all releases available or for your system, you can fetch and install.
E.g. "freebsd-update upgrade -r XXX" results in XXX is unsupported. But returning the list of supported XXX should not be complicated nor difficult.
E.g. "freebsd-update upgrade -r XXX" results in XXX is unsupported. But returning the list of supported XXX should not be complicated nor difficult.