Also not affiliated but my open-source tinycld uses docx as the backend storage for its text package. Supports _most_ of the features (including comments and suggestions) but is still very young. It has a golang backend that reads/writes docx and translates to YJS that the editor reads for multi-user access. Has web/iOS/Android support.

I found docx to be a very well documented format and a surprisingly good fit for this.

https://tinycld.org has a live demo

I went looking around, but I couldn't find why you're making tinycld, and whether I could expect it to keep going as a project in the future.

I expect I could find whether you're using hardened server implementations or reimplementing, but if it's the former, you should advertise that, or if the latter, you shouldn't.