OK, I’ve spent the time to rework everything to use Number:Temperature, and therefore all rules to use QuantityType for working variables, and then some of the tricks and tips from this thread: Working with Number:Temperature Items in rules. This was all substantially harder than I imagined, so posting the link and detail for others who may find this thread and need to do the same thing.
Having done that, I’m still seeing a number of devices not updating. Interestingly, at the moment all the devices that are not updating are connected to one Homegear instance - the one that doesn’t have the piVCCU on it. I’m not sure if that’s coincidence that one is working and one not, or whether actually the thermostats with problems have always been the ones on that same device. I think I recall having problems with thermostats on the other Homegear, but I will monitor to understand.
I will now do some more diagnosis, and potentially run a reboot cycle on everything once I’m certain it’s stable from the data type changes.