Nice job @Betatester!
You can add another backup option:
When used ceemmc to install to internal eMMC you have two options to backup whole CoreELEC partitions.
When booted from external media like uSD or USB:
Make a backup of current CoreELEC dual/single boot installation on eMMC to current used SD or USB device
When booted on internal eMMC:
Make a copy of current CoreELEC installation on eMMC to current used SD or USB device