So currently how I’m managing this is, I send main out L+R to the main mixer. And Sub out to my portable monitor mixer. I connect my headphones to my monitor mixer and turn off the speakers on the keyboard from the settings. When I want to just listen to the keyboard, I set the volume dial to 0. That sends signal only to sub port into the monitor mixer into my ears so I can check/practice. And when I’m ready I just have to turn up the volume, it sends volume out to the main mixer.
What I was trying to avoid is carrying the extra monitor mixer + extra cables + power adapter.