Thank you! :)

I managed to get 4 piezo elements to work, limited by the FPGA. This was actually enough for some reasonable horizontal resolution since I used a phase array configuration, so the downside was the electronics had to generate very precisely timed pulses. The fourth prototype had 10 working elements thanks to replacing the MCU-FPGA duo with just a more powerful FPGA and using conductive glue to more reliably connect the elements themselves.

It was configurable to use any even divisor of 120 MHz, but in practice anything over 1 MHz would not even make it out of the acoustic lens due to the low voltage and low quality impedance matching layer. And much lower frequencies are barely useful anyways, so the true working range was narrow.

For the acoustic lens, I used the parametric design software OpenSCAD, with an equation for aberration-free lenses I had found somewhere and saved long before (will find it if you want) and the speed of sound in the different materials.