"MacOS doesn’t allow a single window to span multiple monitors when the app is maximized or made full screen. "
This is not correct. Parallels for MacOS works as it should in true full screen mode on dual monitors without scroll bars. In fact, running RGS in Windows in Parallels works perfectly too! So it's not a MacOS problem, this is an HP RGS problem. Please purchase and install Parallels for MacOS and do whatever it's doing when going to full screen mode on dual monitors.
I too am having the same issues with the Mac receiver. This is the setup:
Sender installed on Windows 10 PC at work with dual monitor setup.
Receiver installed on Macbook Pro laptop at home with an external Dell P2715Q monitor.
The only way I can get a full screen experience without scrollbars on the Mac's external monitor is by making the PC monitors mirrored or use Display 1 or Display 2 only. Sometimes saving the display settings on the PC doesn't work - it reverts the setting to the orginal Extended desktop setting after aroud 15 seconds, however if I change the settings again and save then it works.
With Teamviewer I have to cycle between monitors using a button on the Teamviewer menu. Maybe that's one avenue to explore?
Any update or timeline on progress is much appreciated!