I had naively assumed that there won't be much difference between one nextcloud (provider) and another. But when I was recently forced to move to a different provider - surprise, surprise - from this moment on sync has been much faster, and this despite the new 'cloud' being free. Since I saw questions about this a few times, here is how migration worked painlessly.
- made a JEX backup
- check the 'fail safe' flag is set
- downloaded the complete Joplin folder (nextcloud turns it into a ZIP container)
here is what you need to know for all current nextcloud instances : nextcloud has no button which allows you to upload a folder, the '+' button only allows upload of single files. Therefore open your nextcloud in a desktop browser, and drag-n-drop the Joplin folder just created (unzipped) into the home directory.
- patiently wait for the upload to complete
- change the sync target in your primary Jop app and sync