So when you say: "unfortunately, there is not enough space on my ssd to accommodate all the vm's..." my m.2 ssd is meant and not the external one... (would have been good if I had written that from the beginning...)

SSDs are basically more suitable for frequent reading than for frequent writing. I think firstly because of this, and secondly because of the price per GB, everything speaks against SSDs as a backup medium. SSDs should be used for the OS and everything else where speed is required (games, video editing, image processing, etc.). For backup and everything else, it's better to buy a large HDD (5TB or more), which fills up by itself over time.