Whether you consider “the simplicity of single files” to be long-term preferable, git probably is your best long-term solution.   You do need to be using text files, though, so that you have meaningful revision-control and differencing capability that is “obviously related to the content changes.”   Do not overlook the revision tracking capabilities that may exist in your word-processor and/or in publicly available systems such as Microsoft SharePoint.®   Your needs are not that far removed from those of ordinary document collaboration even though in this case only one person and one document are involved.