I don't think a 'simple tool' is really possible, in general.
Joplin Server supports End-to-end encryption. So in a common usage scenario, the server doesn't see the contents of the notes at all.
That of course means you'd then have to have more APIs to fetch the decryption keys, your app would have to download the encrypted resources and decrypt them, encrypt them before sending them back, etc.
I assume y'all want an API that would do things like 'add this note', 'scrape that URL' (and I get it, I use it the same way, locally.)
But for these reasons, I don't think it's going to be added anytime soon.
Granted, I'm not a dev, so I might be wrong. But unless Laurent comes out to say it's coming, I wouldn't hold my breath.
Post script: you might argue that in your specific case, you're not using E2EE. In that case, you might be able to cobble together your own API that interacts with the server's DB directly, reading/writing contents in the items table. But it won't be simple, and you'd have to take care not to break anything related to syncing....