You couldn't do separate movement and firing on the keyboard is the thing as the hardware can't detect enough simultaneous keypresses. One of the single joystick modes could probably be implemented though.
Viewing post in Rogue Declan (Amiga) (AmiGameJam Winner) comments
It is possible, I would do this with a heads-up text to put some on the modifier keys and the rest where not more than two rows/columns overlap. All keyboard players know this problem since 8-bit and PC; n-key rollover became popular in 2014-ish and these keyboards could be connected to the Amiga with a supportive adapter. Or they already know some good setups.