Gonna throw in EtherPad. It's been like ten years, since I properly used it, but I believe, it's relatively minimal, so pretty much just Markdown.

There's also lots of publicly hosted instances you can use: https://scanner.etherpad.org/

HedgeDoc also seems like an option that could do some of this. Self-hostable and open source (last I checked).

Nextcloud + collabora online works pretty good for me.

I personally like Cryptpad. It can be self hosted, but you can also make an acount elsewhere considering you don't want the hastle of maintaining.

Overleaf can be self hosted too

How about Git?

I would go for hedgedoc if the team can switch to markdown, otherwise Cryptpad to stay in an office-like suite.

If you already have nextcloud then that + collabora should be good enough. Otherwise installing the whole nextcloud suite just for collaborative editing seems like a lot imho.

Years ago, we used to use Etherpad for that. Don't know if that's still a thing.

If you're looking for something hosted, ProtonDocs is fine, however you'll need an account for each collaborator which could get expensive.

Lots of mentions of HedgeDoc, but it's only for Markdown.

Collabora sounds more in line with what you're looking for. Nextcloud Office might be a bit lacking.

I know proprietary but overleaf seems pretty good at this in my experience.