Apple themselves are just getting around to adding AirPlay 2 to macOS Big Sur, allowing for playback to a stereo pair of HomePods on system level. AFAIK, licensing AirPlay 2 is for hardware only.
I use a few old iPhones I had laying around as bridges to my HomePods: run Roon, activate local endpoint, set playback destination to HomePods. (I automated this with a few Shortcuts and mapped these to the buttons of a presentation remote, so I can turn the iPhone from Apple Music endpoint to Roon endpoint and vice versa with the click of a button).
The signal path does not look half bad (playing Roon > iPhone > stereo HomePods):
The old iPhones stand on little docks, so they also function as little Now playing screens. Selecting the HomePod playback zone on other iDevices enables Now playing (lock screen) controls for Roon. Volume control is done with bluetooth remotes. I wrote a little bit about it here: