Good summary - just one note on GNSS. Pin 14 is for the PPS (pulses per second) from the GNSS so it just needs to be a digital pin so believe we can deal with that one.
@brtaylor
Just a quick follow-up. Hooked up my Ardusimple Zed-9P and loaded up tinyGPS and T4 read the data on Serial2 no problem so looks like you backpacks may work with the T4. Have to set something up more fun like your Kalman code and see how it works. But that's another project - have to make a more permanent cable