I am seeing differing opinions on under what condition to do each type of calibration.

Can someone with more experience please comment / correct my understanding:

Where:

IMU: Cold/cool batteries and drone. Level, vibration free surface. Not affected by "normal" metal surrounds, magnetic or electric fields.
Compass: Away from strong magnetic fields, ferrous metal objects or electricity and away from the controller where possible.
Gimbal: Both the above bt doesn't have to be cold/cool.
VPS: Doesn't matter, just have steady hands.

When:

General: Whenever the app says you should
IMU: After long/rough transporatation or firmware update
Compass: After a large change in location, travelling
Gimbal: Anytime the horizon stops being perfectly level when flying stationary
VPS: After any collision that may cause misalignment, after any warnings about partially blocked sensors or incorrect warnings about prop guards

What have I missed?

Hello Peter, you are correct
It would be best to calibrate compass each time before flight.
Anytime the horizon stops being perfectly level, you could calibrate the IMU firstly, then calibrate the gimbal, and adjust gimbal roll.
Hope tutorials might help:

No need to worry about "metal" or magnetic fields when you are calibrating the IMU.
Just use any convenient level surface.
It's the compass that you need to do away from any magnetic fields.