Looks like PrivateGPT has an endpoint at port 8000, so setting it up is likely going to be similar to Ollama/LiteLLM in the Jarvis guide.
If you want to do it the other way around (manage it externally instead of inside Joplin), take a look at the LangChain / LlamaIndex APIs for Joplin. These extensions can be used to upload all your notes to the local LLM. They do not include logic for updates (on creation / modification) yet.