After some math the ESP32 can not do the audio treatement for this project.
The analog way didn't work and I now miss time to fix it.
So I take 2xADAU1701 devboard, this is a DSP from ANALOG DEVICE which can works stand alone equiped with 2xADC and 4xDAC and easy use graphical programation.
This kind of DSP, I hope, can also do the midi convertion if it still enought memory.
But the consumtion of this DSP is litte high (50mA each) with the 80mA for the IR leds it's the battery life time start to be short (bellow 2h)
For the IR/plexiglass, the 50Hz/60Hz and the consumtion problem, the best way to fix it is to change the sensor.
I try a force sensor like this one :
I install this sensor in a custom brigde polarized with a 100Kohms resistor :
When I play a string the pressure sensor act perfectly as pickup!
The output voltage if far enought and the signal beautiful enought to this project, we don't need the true tring signal but only the frequency and the power.
The next step will be to try the DSPs, the IR pickup work enought to test it.
The pickup with pressure sensor need to redesign the bridge and some try to find the best solution, maybe carbon ink depose by PCB's maker can work for this due...