So the PCB came back and I proceeded to stuff it and do some basic tests - not such a complicated circuit really, but wanted to be sure I could switch on/off the Rpi Zero W power with the USB load switch. Check.
I stopped dithering on the tuning knob encoder and realized that I'd free up a ton of space on the chassis deck if I merely replaced the tuning condenser with a pot. I took a scrap piece of aluminum and bored a hole for a potentiometer at the right position and that was all it took. So, now the tuning dial moves 300-ish degrees, the same as the pot - and the angle is just a very simple voltage reading from the ADC. Easy-peasy.
I removed some no longer used original components from the radio chassis underdeck, too. That made space for the 120V to 5V supply that will feed the retrofit PCB with a constant 5V. More on power domains in the following log entry.