Cheers Heckert - yes it's definitely trickier compared to the mouse, even with a generous dead-zone for the right stick. I was thinking about how to add alt-fire on the right stick and indeed it'll need to be in combination with a trigger or button press. So I may switch the main fire to those controls too. I guess I was using Arcade / SNES Smash TV as a reference point for how to handle the shooting.
All that said I did find the thumbstick fire a lot more visceral and feeling like there was a bit more skill involved rather than just clicking.