I use Cryptpad for collaborative document editing/sharing and I've been having a good time with it. They support rich text documents, spreadsheets, slideshows, diagrams, all the usual office suite stuff. You can share documents directly with other users, or generate shareable links you can send people, and you can choose who gets to view vs edit stuff. The server software is open source and self-hostable, there's a main instance run by the developers at cryptpad.fr but there's other ones out there too.
All of your documents are end-to-end encrypted, so fundamentally, the admins can't see any of your documents unless you've chosen to share a link with them (or share a link somewhere public where they run across it). The cryptpad.fr admins also specifically adjusted their ToS a couple months ago to allow adult content (there was never a deliberate prohibition on it, just an unintentional side-effect of citing their developer community guidelines as part of their platform ToS). From my past conversations with the admins, they're comfortable having kink on their platform; I'm trying to get a little more clarity about their stance on noncon kink stuff in particular, since the updated ToS are still a little ambiguous there. But again, fundamentally the admins have no way of knowing what's in your documents unless you share a link publicly, so honestly I'm not especially worried about the exact limits of their ToS. There's also a list of other public Cryptpad instances, which have their own admins and their own policies independent of cryptpad.fr; looking at the ToS for https://cryptpad.disroot.org/ and https://cryptpad.jpeg.gay/ to pick a couple at random, neither of them have any restrictions on kink fantasy content, as long as it's not harassment or hate speech or illegal.
In general though, regardless of what you're using, seconding the recommendation to always have backups of your work in multiple places, locally on your own computer and elsewhere. Cryptpad has a button in Settings > CryptDrive you can use to bulk download all of your documents, which is worth doing regardless of how friendly a platform is, just in case of outages or whatever else.
From talking more with the Cryptpad.fr admins, they're very emphatically in support of adult content creators and sex workers, and they updated their ToS to clarify that what they prohibit on their instance is "content promoting real-world physical and/or sexual abuse". They're good people, definitely check out https://cryptpad.fr if you're looking for someplace to host shared documents! And if you've got the leeway, toss them a few euros to get some extra storage space and help them develop the Cryptpad software further <3
