Hey Tom,
you have a lot of suggestions for probable causes in the other thread. Here, for me, the information that you are using the machine since one year (and that it’s not dependent on router or spindle) immediately triggers a strong association with the well-known “Curly Z cable is not made for permanent motion and shows signs of internal hidden cable breaks resulting in sudden unexpected Z moves or steplosses, that usually appear after one year of use” issue. Could also be the Molex stepper sockets on the left of X gantry which are subject to permanent motion too (and are not made for this too).
Also even with many nodes I believe it should not loose steps or erratically lower Z axis