I use Zerotier, which is basically an easy VPN that is cloud run and allows hosts to connect (and game) to each other over secure connectivity. This is usually a trivial installation from AUR packages with Arch, but as you say the repos are not installed as an "image". Since these user kernel drivers via VPN tap/tun functionality, it's not friendly toward Flatpak installation, and needs done in the base os (I think, there isn't a flatpak for them).
There's a lot of easy-ish VPN's out there like Zerotier or Tailscale that allow free use, or more traditional VPN's including OpenVPN, Hamachi, even enterprise like Fortinet FortiClient, Palo Alto Globalprotect, etc. It would be nice to account for installing these in the base os as well via their AUR packages.