What is the correct way to save my configuration before I try to install any of the omv-extras?
I personally use clonezilla for that purpose. Before I make a major configuration change I create an image of my OMV system disk. Indeed it is a little bit laborious and needs to shutdown the Nas. But if something goes wrong I have the possibility to go back to an earlier state and can start another try. The usage of clonezilla is quite easy.