A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
PowerPoint's design doesn't make it easy to force adherence to brand guidelines without any user education. Your UI mod/VBA macro idea is as close as you're likely to get to a solution. The UI and VBA will require installing an add-in on every computer. Consider using PowerPoint events to make revisions while the file is open instead of when closing it. Here's a page with more about VBA events: PowerPoint Application Events in VBA The page is a bit out of date but not much has been changed since Office 2016.
You'll also have to restrict access to themes and templates from Microsoft and other sources.