Thanks for everyone's comments - such a great community here

The volume pedal is a good idea but maybe more so for a regular keyboard. I'm playing a Roland AX Synth keytar and the volume knob is on the neck within easy reach. My left hand is usually not on the keys so adjusting the volume is not too hard. The trick will be to make sure I'm hearing a FOH mix so that I adjust correctly!

I can also set a custom volume level for each patch so I'll do that too to try and minimize differences between patches. That won't help with solos though - I'll still need to up the volume knob for the solo and drop it back down when I finish the solo.

I *could* save the same patch into 2 different quick slots, one with normal volume and one with a higher volume for soloing. Only problem is, the keytar only has 16 quick slots (2 banks of 8) so I wouldn't have enough slots. Alas, there is no provision for saving a setup for each song on the keytar - my 16 slots have to last me for the whole concert.