I would stay away from YAML (almost at all costs).
For us to be able to help you, I think you need to give us some more details about the organization of your work. If each chapter is a separate text file, then you don't really need to do any branching at all (assuming people will only make changes on their own files and not touch others' work). If it is a single text file, branching (or anything else, to be honest) will not help much.
I assume chapters will have their separate files. As long as you can control who touches which file, everybody can work on the same branch (also referred to as "trunk based"). But if you fear that people may interfere with each other's work (willingly or by accident), then it makes sense to create a branch per chapter to keep contributors at a distance from each other. But working on a single repository requires some sort of an agreement on the workflow.
Don't they already have the names Leap and Tumbleweed? Changing the name to Leap would make sense since it's the name of the "official LTS" version. At this point it sounds like "openSUSE" is the name of the project and not the distro. But I haven't been following them closely, so perhaps I'm wrong.
The only such transition I was involved in was switching from TFS to Git, and there was no discussion. It was the obvious thing to do and for a while we needed to support some developers who are new to Git.
So, it all depends on the type of change you want to implement. Most people don't think much about a piece of software being open is significant. That's why the main selling point should be the product itself. Especially in organizations openness alone is not a strong enough argument.
But with individuals, it may help to inform people about FOSS instead of just suggesting alternatives ("Do you have a moment to talk about our lord and savior Stallman/Torvalds?"). If the individual doesn't understand or subscribe to the values, the switch may be temporary. My 2 cents. Hopefully others will come up with better tactics.
This problem is now solved for me. I didn't do anything. Either a Firefox (current version 137.0.2 on mobile) or a Lemmy update (current version BE: 0.19.11) seems to have solved it.