Part of the Furniture
All sorted! Turned out to be a VPN Client routing conflict when using 2 clients both on TCP 443 (NordVPN). Got bitten by this myself in the past! Switching one of the clients to UDP and all is now behaving as expected, with LAN going out on one VPN client and YazFi pushing an SSID over the other. I'm pleased to announce that thanks to @Brenneke and their patient testing, we can confirm the script works with the AC68U!