I have a single-page app with all reusables…and a lot of floating groups…
but I have all my floating groups on the index page, and then send data to them from the reusable element. This method works fine and is very easy to set up.
The z-index for the floating group will depend on the parent element (in this case the reusable element). I have a similar setup but they are stored in a Header reusable which itself is a floating group.
Also note that you can manage Overlay z-index using the element tree. Anything at the bottom has priority.