@Oliver@DSunY I've just PMed you asking what return address for the faulty board, Demi never sent this through to me on Aliexpress chat. Happy to return the faulty board to you for testing.
Im 100% certain this isnt a 'wifi configuration' issue. I've been working in IT as a programmer and UNIX system admin for over 20 years, and I know how to configure wifi :).
I've had the faulty and working board hooked up to TTL serial on the debug port, the faulty board is completely dead after it prints an initial boot string of "V:", the working board prints "V:BK7231N_1.0.1" (and continues booting with lots more output).
I'm not very familiar with Tuya devices but if I had to guess it might be a corrupt boot loader.
I had holidays in late January away from my tank and have had travel for work every few weeks since then. Before I left I switched the lights over to the old controller and I havent found time to swap them back to wifi, so the replacement board I got in Jan is still in the packaging. I'll make an effort to get this done soon.
(I also picked up a second hand Cade 1200 S2 tank in March so I'd been planning to do the wifi swap when I move my tank over).