I did some more studying, and came up with the following table:
I hope this is right!
Now, I'm not using the M5Stack bottom board which has the PortB and PortC, so I figure that I can use the RXD2/TXD2 pins as well as the G36 and G26 pins. I'll use those for the other 2 SoftwareSerial ports, and G27 and G19 for the other pins.
The thing that I found confusing was that they do list the pins in the M5Stack documents, but they don't really explain what is available to the user. This took a lot more work than it should have!
Sir Michael