I made a small expression box (a Hammond 1590A box) with an expression pot and a stereo jack input (TRS). I go mono only and split the signal on the grid into two separate lanes with OUT1 and OUT2. The main out to the PA is on OUT1 and the volume is controlled by the main volume knob on the device.
OUT2 feeds my personal monitor on stage, if I need one. The expression knob is plugged into EXP2 and assigned to the OUT2 lane output and the level is turned all the way down.
Once I set the main output to the mixer, I can easily dial in the volume of my monitor and make quick changes on the fly. This works flawlessly and can also be used to go to a power amp and cab on stage or to a monitor mixer etc etc.
Naturally this will not work if you require both EXP inputs for actual pedals to control other parameters. I’m happy with just a wah though