I would very much like to play Turrican 2 again, however it would be very tedious to play without autofire, yet you need to be able to disable autofire on a regular enough basis that going into the WinUAE menu during gameplay would be tedious.
Is it possible to set a button on my joypad to purely toggle autofire and keep the main fire button as... the fire button?

The 'toggle autofire' option uses the fire button to decide when to start auto-firing or not. On an Amiga gamepad one would use a switch to disable autofire when necessary.