Looks a lot better since I installed the latest v10.3.0-rc.12 update yesterday:
v10.3.0-rc.12
Core
- Adds optimisations that reduce the amount of data written to persistent storage.
- Updates translations.
Zigbee
- Fixes an issue where Zigbee would stop working when Thread started.
Not sure how this issue went unnoticed by Athom for so long, if it was caused by excessive writes? It was already mentioned here: [Modding][HowTo] Investigating Homey’s eMMC wear estimation - #20 by Sharkys
[UPDATE 11/02] Today the high average CPU utilisation unfortunately returned after 4 days. tries restarting some apps but that did not help, a reboot did resolve it for now… 
[UPDATE 16/02] Fortunately after the restart last time the high CPU usage did not return yet so overall it still looks promising.
