Allow me to share my observations here:
Long story short: I just moved from Nimbus Notes / Fusebase to Joplin. I read a lot about data migration from Nimbus Notes, and came across JIMMY. But I learned quickly about the mass export problem with Nimbus Notes. Here is the thing:
There still is a Nimbus Windows-client that can do bulk-export! However it is hidden on Nimbus' official website, looks like it was forgotten...
.
This is not my discovery, I give kudos to user @palashman on Github, who mentioned it in a discussion.
Go to https://nimbusweb.me/nimbusnote-offline.exe
I easily bulk-exported all my Nimbus notes in one go to HTML. Nimbusnote-offline.exe has the menu item "File/Export all pages", which is not available in the client that is linked on the Nimbus download page ("nimbusnote.exe").
Then I used Jimmy to create markdown files for Joplin, and I say goodbye to Nimbus/Fusebase!
The only observation in Jimmy was: with "--frontmatter joplin" argument, I only got the note title, but not the creation date of the original Nimbus note. But it might be that this hasn't been exported at all. All other screenshots, pictures and attachments are there, and are shown in Joplin where they should be.
Thanks a lot to @Marph for Jimmy!