There are probably as many workflows being used as there are people. So keep this in mind that no one single workflow is the best.
I personally prefer git to share libraries and project over dropbox/gdrive.
As for the KiCad setup I also prefer using only and only global libraries. As others have said it is common that symbol/footprint used for the first time is likely to be used again. So for collaboration this requires that everybody has the same set of libraries which are shared through git. Obviously any new symbols/footprints/3Dmodels have to be added to the libraries before using them in the project. We use similar procedure that KiCad uses with official libraries. This slows the development of the project a bit but on the other hand every symbol/footprint/3Dmodel is properly reviewed before being used for the first time. So if everybody is using the same set of libraries then the collaboration is much much easier.
As for propagating the changes done in one project to other project, again git with CI pipeline. It would be possible (I haven’t done this) that when a project is pushed to remote repository a CI pipeline is activated which checks if all the symbols and footprints used within the project are the same as in the libraries. Also a CI pipeline could be triggered upon global libraries change, which would check all the projects and report the differences.
Regarding collaboration on a project I know that collaboration on schematics is doable if each person has a dedicated hierarchical subpage(s) to work on. Then you can merge contributions pretty seamlessly without any external tools. Obviously this is not straight forward with layout. But (and this is a shameless plug) it is doable with Save/restore action plugin. If the design permits, you could layout one (or more) hierarchical sheet and export the layout via plugin. Then one has to import all the layouts done, and place them together.