For what it’s worth, I believe Tailscale on the Apple TV blocks IPv6. In fact, if you activate Tailscale on your HomeKit Hub, it has the side effect of all thread devices no longer responding until you disconnect from Tailscale and reboot the Apple TV. It’s a major pain in the ass for my small HomeKit / Thread setup.
Here’s the corresponding bug report on GitHub.
Based on your stated assumptions about backdoors, how about building your own RISC-V phone?
https://mikrophone.net/
It’s not in a very usable state, but the hardware is open source.