Under Content / Edit I have an entry called Homepage.
Under Design / Templates / Edit / Home I have two entries… index and homepage_sandbox (which are identical except they use different global embeds for a header).
How can I have a separate version of the home page, using a different template, so I can preview it in a browser? I need to be able to work on a copy of the home page before making it live.
I usually do basicly what you have done. If my dynamic content is not changing & only the template, I just usually Duplicate the site/index.html Page Template and I usually name it New.html but your homepage_sandbox.html works as well.
I then access it through https://mydomain.com/site/homepage_sandbox