I'm slowly coming back to spending more time on this project. Progress could be faster - I still have to work on my time management skills, and maybe stop doing additional research at this point as it's taking too much time right now.
Breadboard wiring errors
Just as I was preparing the components for the hardware prototype wiring, I noticed some errors in my Fritzing designs - thankfully I noticed those before connecting and powering up everything. Apart from one missing GND connection line on one of the buttons and another missing crossover line for the 3.3V power line, I accidentally mixed up some power and ground connections - which could have been disastrous if I hadn't caught that before the fact.
I have to admit that this is the first time I'm working with a standard plugin breadboard as well as the first time I'm working with the Fritzing software. Good thing the physical breadboard has some nice plus and minus signs for the default usage of the main lines along the long edges of the board...
I fired up Fritzing again and corrected the errors. Here is the new version of the prototype layout using the Raspberry Pi model B+/2/3:
And here is the same corrected sketch with the Raspberry Pi model Zero:
In order to recreate this setup using real hardware, I need to do some basic soldering on a few components. Otherwise I won't be able to connect them to the board. Then again, once the setup is working, I'll have to do more soldering for the final prototype anyway.
DAC option: PiMoroni pHat DAC board
The incredible has happened a few days ago: I've finally received the PiMoroni pHat DAC board which I ordered quite a while ago (a litte more than 2 months if I remember correctly - I could look up the exact date):
While I've already incorporated the Adafruit MCP4725D DAC breakout board into the current hardware design as an alternative DAC and audio output for the Raspberry Pi Zero, the pHat DAC board has the same form factor as the Pi Zero, and could possibly be a little easier to set up and use.
The downside - as far as I can see it so far - is that the wiring connections to the other parts like the switches, resistors etc. can be a little tricky to do with the board connected to the Pi Zero trough the standard headers. I may have to add the additional wiring either to the back side of the Pi Zero or to the pHat DAC board on top, or come up with a kind of t-junction solution for the necessary pins. Or maybe using some "flatband" cable just for the few really necessary connections between the Pi and the DAC board, and single cable connections for the switches etc. is the better option - if anyone has worked on similar wiring options in the past, and can share some advice and experiences, please add them in the comments...