The Kickstart 3.2.2 roms arrived today, and I swapped them on the board.
Oddly, these EPROMs are 1 pin too long for the A3000 sockets. The seller clipped pins 1 and 42 to suggest the correct alignment.
Now I can tell the Z3660 to use the on-board ROMs and... IT BOOTS!
This confirms that the original ROMs had indeed gone bad, while Gary and other bus logic still works... which is very lucky 😅