Thank you for your willingness to contribute to our #earlyaccess testing, and please accept our apologies if the switch to the Beta group has rendered your system inoperable for nearly two weeks.
The team is actively testing Starlink compatibility with IPv6 and I’ll merge your detailed notes there shortly. The details you’ve already provided are immensely helpful as we troubleshoot, so if you’re eager to get ARC functional in the meantime, I recommend the TailScale workaround here: Tailscale implementation with ARC to circumnavigate ISP CGNAT
The above thread concerns Synology users, but for a Mac user like you, there are also instructions here: Setting up Tailscale on macOS · Tailscale. It’s a simple app for your Core and Remote (phone) that circumvents port forwarding. Put simply, it will safely extend your local network to the phone outside the home.
In the meantime, we’re working with other Starlink users to work this out. I’ll report on progress in the main IPv6 testing thread as soon as I have an update: Testing IPv6 support in Roon (for users locked out of ARC due to ISP reliance on IPv6 or CGNAT)