So this was a relatively easy upgrade but it is important. The crankset is what delivered the relatively limited power of the rider to the rear wheel allowing the bicycle to be more efficient (most of the time) than walking. So it was to the XX1 crankset and its creaky bottom bracket so I could run my beloved XTR Trail with Stages Power Meter crank set.
I also installed a Blackspire 32T Snaggle tooth chain ring, their version of the new narrow/ wide format, in order to encourage chain retention (but obviously forgot to take a picture of it).
This is not revolutionary but it does move the Garmin to a slightly more protected position and allows me to record all my riding data again. The knowledge that my bottom bracket can manage a 40 km trail ride without creaking or a crank arm falling off is important too. I am still playing with bike set up and mainly trying to find the right pressures to get the Monarch Plus Debonair to provide traction and provide full travel!