Not quite sure what kind of privacy you need, but matrix is reasonably easy to self host, and allows you to federate if you want.

Also has plenty of clients.

If you want low tech, hosting IRC is easy too.

They are currently adding calendar support, which is the most feature for me. We are looking for work it’s integration.

The only way I know Forsthaus to work, you would need to use something like the playwright mcp, and use it to let the LLM view what you are browsing. Not sure that even has a GUI.