All of these features are helpful IMHO. As others have said, I don't think there is a lot of demand, and there are already viable options on the market.
One comment...IMHO more notch filters would really be helpful. In my experience you don't hit the point of diminishing returns until you have notched out 4-6 problematic frequencies. Notching only one frequency is often barely an improvement.
My solution was to use a Ashley MX508 rack mixer. For context I switched between three instruments, so two channels was not sufficient. This option was a good choice for local gigs as it was housed in a 16-sp rolling rack with a power amp, rack drawer, tuner, and Furman rack power module with lights. Way to big and heavy for one person to load in and out of a POV. It worked great for touring, where all band gear was transported in a cargo truck.
This mixer does not have instrument level inputs, so I used Shure A95U line matching transformers to convert the signal to mic level. I am pretty sure the resulting input impedance was way too low for the typical piezo, but I didn't really notice any degradation to the sound of my Realist (original version).
The channels strips on the Ashley only had Bass, Treble, and one variable frequency Mid control. Not quite enough processing power IMHO but it worked fairly well. If I were still using this rig I would add an Empress MkII ParaEQ Deluxe for the upright. This would increase the load on the piezo to 1Meg and provide a variable HPF and additional EQ that can be configured as a couple of notch filters.
A current mixer I would research is the Behringer Flow 8. Digital mixers tend to have way more processing power than analog. Input impedance for the high Z/instrument level inputs is 2 meg. Hook the Flow 8 up to your iPhone and a powered speaker and you might have a really good doubling rig. This is a suggestion to put this on your research list, rather than a recommendation to buy. I have not use it and I have not fully explored it's capabilities or suitability to the task.
