I used to use Nextcloud with WebDAV for this purpose. That was before Joplin made their own server. I have since moved on to Notion, though.

Joplin can use WebDAV syncing: https://whhone.com/posts/webdav-syncthing/

I use the docker version of the Joplin Server, it syncs with everything I’ve thrown at it which is a laptop, my Android Phone(s) over the years and my Desktop with no issues. It’s super basic so you just set up an account in the self hosted server and then proceed to login using the Synchronization settings in your Joplin App.

If you have any existing data, Make sure to Back it up First!

This is my compose I use. Sub with your values obviously.

services:
    db:
        image: postgres:15.4-bullseye
        volumes:
            - /mnt/appdata/joplindb:/var/lib/postgresql/data
        ports:
            - "5432:5432"
        restart: unless-stopped
        environment:
            - POSTGRES_PASSWORD=postgres
            - POSTGRES_USER=postgres
            - POSTGRES_DB=joplin
    app:
        image: joplin/server:latest
        depends_on:
            - db
        ports:
            - "22300:22300"
        restart: unless-stopped
        environment:
            - APP_PORT=22300
            - APP_BASE_URL=https://public.url.example.com/
            - DB_CLIENT=pg
            - POSTGRES_PASSWORD=postgres
            - POSTGRES_DATABASE=joplin
            - POSTGRES_USER=postgres
            - POSTGRES_PORT=5432
            - POSTGRES_HOST=db
            - MAILER_ENABLED=1
            - MAILER_HOST=yourmailhost
            - MAILER_PORT=587
            - MAILER_SECURE=0
            - MAILER_AUTH_USER=Your Email user
            - MAILER_AUTH_PASSWORD=Your Super Secret Password
            - MAILER_NOREPLY_NAME=Your No Reply Name (eg:  Noreply box)
            - MAILER_NOREPLY_EMAIL=no reply email address
            - TZ=America/Los_Angeles 
volumes:
  joplindb:

And then in the settings for the sync in the app:

To get there, Go to the Tools > Options > Synchronization

Hopefully this will help. :)

Certainly. This is what I do:

1. Set up Syncthing normally
2. Within the folder that you are synchronizing with Syncthing, create a new folder called joplin or something
3. In Joplin (on desktop, mobile, and every device), set the synchronization target to your local filesystem.

• Configuration > Synchronization > Synchronization target > File system

4. Point it to your joplin folder that you created in step 2

• Configuration > Synchronization > Directory to synchronize with (absolute path) > /your/path/here/joplin