Something very odd went on, not sure what (tried different power up modes, different boot modes, just wouldnt take the input from SFC30).
Just wrote RetroPie 4 final to a card. Chose basic install.
Added controller with hack off, added udev rule, left the boot mode where it was, didnt choose "boot" or "background".
Turned controller off after it registered and connected.
Rebooted.
Waited until fully in ES, turned SFC30 on, connected automatically.
ES happily saw its inputs - wrote the correct values. All power ons were with Start+R here.
All good! :)
Now just trying to work out how I managed to get ES to not see inputs from a SFC30 but see the FC30 Pro....