When it comes to self-hosted alternatives for collaborative document editing, consider tools like Nextcloud, ONLYOFFICE, or Collabora Online.
Nextcloud's Office Built-in Integration with Collabora is terrible. It sometimes works and mostly dosen't . You may get lucky hosting a separate Collabora server and integrate it with Nextcloud that way versues using their built-in solution
It's essential to find what works best for your needs, and hosting a separate Collabora server might indeed offer a more reliable solution.