What are good platforms for collaborative writing with multiple senior authors with limited technical background?

If you are in a hurry, you might want to try an online TeX editor such as Overleaf.

Another solution if you are using TeX would be to work alongside a version control system, e.g. git. Keep in mind that it takes some time to get used to it, though.

I do not know about non-TeX-solutions but i feel that a plain-text format has a couple of advantages for this kind of application.

I am looking forward to read other responses to learn about other options.

I use the following myself:

Non-technical: Google docs. Low threshold, can do basic stuff, cannot view history. Main drawback: No version control. (Also my university in principle does not allow it, but if you promise not to tell anybody...)

Technical, many collaborators: Overleaf. Can do full latex, not straightforward to go to commit history, but can be done since it is git based. I am not too fond of their interface, find it a bit clunky, but many of my collaborators really like it, so I use it anyway.

Technical, few collaborators: latex documents on private gitlab instance. This is for me the best. Commit history easily accessible, anything you want to do, can be done. Gitlab even has a builtin IDE, so you can do small edits directly in the browser. Drawback: high threshold.

This, for me, covers all use cases.