It looks like the M1XSP is sending the correct Aprilaire commands on startup. You probably won’t see the rule triggered commands from the M1XSP because the 8811 isn’t connected. The M1XSP will need to receive a reply to it’s poll from the thermostats so that is knows that it is there before it will send the commands from a rule.
The null modem test proved that the M1XSP is working and it’s set to Aprilaire mode. Connect it back together and try one more time. Power down both the M1 and the Aprilaire system. Power up both and after a couple of minutes check on the keypad to see if the thermostats are enabled.
Might want to start by having only one thermostat (address 1) to see if that one gets recognized by the M1XSP.