@scavy slight update, I gave my (eneloop pro) batteries a full charge and it now managed to boot part the setup screen.
I don't get the installing gpi case screen any more, so not sure if the problem is permanently resolved or if it will start failing again when the batteries run down a bit, could be that now the safe shut down is executing it doesn't need to do that particular pay of the boot sequence any more
I've just configured my GPi Case/Pi Zero 2 W and it won't boot on battery power - seems to get stuck
in a loop between the initial "have fun with recalbox" screen and the "installing GPi Case" screen . So, has to be plugged in to USB to boot up and then can be removed from the power connector and will run on batteries quite happily.
So, I guess, some part of the boot process draws just a tiny amount too much power and causes it to go into a bit of a meltdown.