Can I use the Mini to replace my harman/kardon speaker-set which I use with my iMac? Did that work with the “old” HomePod?
I don’t see why not. macOS should let you select any AirPlay device as an audio output device.
You can use a HomePod as audio-out for a Mac.
I believe Catalina (and later) use AirPlay 2 meaning that there should be no lag.
However, doing so will interfere with using the HomePod for playing music independently, for example.
Also, you cannot setup a HomePod stereo pair as the default audio out for a Mac, although you can send audio from specific apps (like Music-née-iTunes) to a stereo pair.
That’s what I suspect.