I see a good few topics on this and nothing but complexity. I would like to suggest that Nextcloud implement a user migration feature. The use case is is simple, as a user I decide I want to use another instance of nextcloud, I have files, contacts, calendar, tasks … and I’d like a simple download option from instance 1 which downloads everything of mine and a simple upload option on instance 2 so I can upload all my data.
There’s some devil in the detail but not much IMHO, it’s a simple and nice feature at some level that makes migrations easy and for that matter user specific backups (I can back all my stuff up).
One detail devil is that all my user info is relatively compact bar files which can be massive. And so the download could clearly make that if not every part, optional to download. File in particular are optional as I have them all on my PC from the desktop client. Now all I’d need is after importing my stuff, to point my desktop client to the new instance and account and it reliably push everything up to the cloud.