Does the server have to run headless, or can one person still run games locally on it while another has a remote session? That is, would I be able to play on the monitor/mouse/keyboard directly attached, while my partner has a session on a laptop?

Very cool!

When you specify To: localtesting@aussie.zone how does the bridge know if you meant https://aussie.zone/c/localtesting or https://aussie.zone/u/localtesting instead?

Maybe everyone else is seeing something I am not, but what routing path is for internet traffic to “Future PC” and its neighbors? You have the ISP modem labeled as a passthru, which means it is not handling NAT/firewall. What device is binding to the WAN IP address that the passthru will be handing out? An unmanaged switch is not going to do that.