Hi, when we started as one of the first signup partners (july 2018), we requested exactly this feature. It took quite some time, but GitHub - nextcloud/backup: Backup now. Restore later. · GitHub was the result but it lacked a lot of functionality. When it got up to speed, it missed compatibility with newer versions. It also had problems with a lot of storage as it dumped everything in 1 file, with timeouts while writing the file as result. It is also not on the supported apps list of Nextcloud so it’s community maintained and also not on our list of apps we would recommend unfortunately (even though a lot of effort was put into the project). So we keep track of it but currently tell people to copy their files, make exports from the groupware and don’t migrate the database. Or we create a database backup on request, but not all vendors accept importing the database (we do accept importing a db btw).
So it’s kind of a story but to answer your question: there is basicly not a (by Nextcloud) good way to do an export/import like you want to do. We have asked Nextcloud a few times to ask them to support the app, but alas.