I have looked in last time through the code of the NcVM and I guess that it will make a lot more problems when upgrading that way. You realy need some knowledge about the insites but no rocket science 
Why you don’t simply download a new 40GB image and migrates your nextcloud server to the new VM. Thats what I would do. Maybe you can even simply reuse the data disk.
But with all the other steps you are mainly on your own or you can buy support from HanssonIT.
What I can offer: this script to upgrade your php Version wthin the virtual machine without a hassle so that you can upgrade your nextcloud to the latest version and you get 11 more months to migrate your cloud to the new VM.
Much and good luck,
ernolf