I added the firmware and Petitboot showed on the GPU again, but then the main OS didn't.
So I removed it.
I either needed to change the fastreset setting, or 5.5 initializing the card doesn't play well with 6.6.
I'll try again at some point in the future but as I say I can use VGA or ethernet for now, I'm just happy the card works with the main OS again.