Actually it's really simple. I did this myself. First set up an empty sync'ed directory on all your devices. You can test it by syncing random files but when you're ready to go have it empty.
Then point one Joplin instance at the empty folder as a "local filesystem" sync target. Joplin will populate the folder automagically with the required files. Then wait for Syncthing to do its sync thing, until the matching folder on your other devices is populated, then point those Joplin instances to their local filesystem sync targets. You should then see your notes from the first instance of Joplin appear in the others. Voilà; you're good to go.
If you want to change sync backends, all you have to do is change your sync target in Joplin and enter the credentials (example: Dropbox username/pw). Joplin will again create a folder in your cloud drive and populate it with the files Joplin wants to be there. Then point your other Joplin instances to the same cloud drive. Couldn't be simpler.
Edit: If you're in the situation where you already have different notes in different Joplin instances, and you now want to sync them - I have not tested this edge case. But I expect it to be handled elegantly. For example, Syncthing will look at timestamps and simply create duplicates when un-resolvable conflicts arise. Then it's up to you to sort them out.