CryptPad - open source private collaboration on documents, polls, kanban boards, whiteboards and source code editing
Yes, EtherPad also does collaborative real-time editing on a document but the difference here is local encryption taking place on the client side and quite a few other services also rolled into one eg. whiteboards, source code editing, Kanban boards (like Trello), polls and even simple presentations.
You can sample a poll by going to this link and voting on whether you like CryptPad or not - https://cryptpad.fr/poll/#/2/poll/edit/1+7vPrTX2kfwIOzDJk5NaAVX/embed/.
Self-hosting, of course, will remove any file storage limitations and give you even greater peace of mind over security.