Hi AL
My initial thought was whether there could be a synergy between your upgrade methods and the solution https://discuss.moodlebox.net/d/18-upgrade-moodlebox/3 Nicolas is working on. Thanks to your write up, I see why it wouldn't help much: MoodleBox is a single image, your Pi cluster consists of many!
Anyway, if you have time please do write a blog or something with more details. Lot of RPis go through my table, I would love to experiment with a Moodle cluster of RPis.
Hardware and performance
Cluster setup Help
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