For conversation with beta readers, I usually do DMs over the platform where they usually do their beta work stuff, which these days tends to be Discord but I've had some cases of PMing over topical forums's message systems.

For editing docs, my usual workflow is I set up a cryptpad or etherpad version of the document for them to edit and annotate and then we compare notes. Other times I use my own hedgedoc instance when what I want is light notes or minor fixes, or I just e-mail the document over.

I've been looking for some sort of fandom-established privacy-respecting Google Docs alternative but so far no one seems to have done it, which is strange considering using Google for that kind of stuff give s enough ammo to some evil companies (ahem... Nintendo) to sue two or more people in one go.