After reading many comments I decided that RasPi Zero + reSpeaker 2-mic HAT seems a more affordable combination in a tidy little package. It is.
I have currently got 2 satellites running on my desk. they are:
- RasPi 3B I already had, with the reSpeaker 4-mic HAT.
- RasPi Zero WH with Adafruit Voice Bonnet (equivalent of reSpeaker 2-mic HAT).
Both are running Raspberry Pi OS Lite and Rhasspy configured as a satellite with identical configuration (Audio Recording, Audio Playing and Porcupine WakeWord detection with the real work done on a RasPi4 base station), and both do the job well.
Of course the RasPi 3 is faster than the RasPi Zero - most notably in booting up and starting Rhasspy. My RasPi Zero just took 0:55 to boot to command line, and 5:48 to start Rhasspy to the console message advising that the web server is started. This delay got me looking at another add-on to use the LEDs to indicate when Rhasspy is ready.
Once running, Rhasspy on the RasPi Zero is only fractionally slower to recognise the wakeword and respond. I judge that it is still very acceptable.
If I build another satellite, I would however use a RasPi 3A since it has the same RasPi 3 processor and Wi-Fi, but saves cost by having RasPi Zero’s reduced I/O.
I would also suggest making a dedicated “RasPi Zero Rhasspy satellite” image with trimmed down OS and limiting the Rhasspy options.