This one looks like it might be suitable. It seems like you can make your groups for each Distro.
I can’t say I am a fan of Discourse from the little I have seen so far.
I can’t find how you start a new group or if they are already static. It does have an Ubuntu Group already that says it covers all flavors:
Ubuntu and derivatives
For users of Ubuntu and its derivatives.
Join this group if you’re using elementary, deepin, KDE neon, Kubuntu, Linuxfx, Lite, Lubuntu, MakuluLinux, Mint, Nitrux, Parrot, Pop!_OS, TUXEDO, Ubuntu, Vanilla, Zorin and others built on Ubuntu.