I’ve got 14 of the Aprilaire 8800 thermostats going through the 8819 panel then through the 8811 converter. I don’t know if the 8811 is required, but I assume it’s doing some RS485 “translations”.
I updated from the 8870/8818 panel to the 8800/8819 panel prior to the 8800 going out of production. The 8870’s don’t have the touch screen and look older and they seem to be easier to find now what both the 8870 and 8800 are out of production. The 8818 panel I though was better with the LEDs, and I didn’t like that the panels are cross compatible.
I think that you should be able to run 2 different M1XSP, but that would be a question for Elk.