I used to do exactly that a long time ago with Syncthing and it was as simple as you say, but it always broke something, but after a while it was literally impossible to do and after some research on Joplin's Github issues I found out that due to a change Google made in the Android storage system it was no longer possible and "they were investigating how to fix it".

In the end I got tired of waiting and pay a cheap Nextcloud hosting to sync with webdav and that's it.

I think you’d need to sync it to the android client, rather than import it. I self host Joplin server for my sync backend, and use the app across my mobile and desk computers. I don’t love Joplin, but it’s the best self hosted, private, end to end encrypted notes app available on all platforms that I could find.

When using Joplin using the built in sync features is the way to go. Trying to sync files manually or via something like syncthing will just result in pain. My family and I have personal and shared notebooks all synced via my self hosted Joplin sync service and it works swimmingly. Before I set that up I used OneDrive and that worked too, although a bit slower.

If people are wondering why Joplin has such weird back end files it’s because Joplin keeps note history. Convenient, but makes for a messy file backend if you peak behind the curtains.

You have to sync folder that includes notes by choosing "file system" option in sync page in settings. You don't need to import it. Sync notes to some folder on PC, transfer this folder to phone and sync it there.

I could never understand the fascination with Joplin. I’ve tried to use it multiple times but I just don’t like it. Sync issues, idiotic file storage, etc. were never worth it for me.