We can't use #Overleaf, as the other contributors don't speak LaTeX.
#Etherpad is less bloated, but they also don't support auto-formatting with Markdown or something similar, so I have to use a mouse to define the paragraph style (paragraph, heading 1, 2, etc.).
Editing plain text files locally and sync them with git is not an option.
Moodle has a Wiki option, but no two collaborators can edit a page at the same times. Oh, ☎️ the 1990's are calling. They want their software limitations back.