You don't need to copy the entire folder.
The overrides system lets you only copy the files that you make changes to.
So for example ... if you don't need to alter the look/feel/functionality of the login page, then ... you don't need to duplicate the login page files.
If you're going to customize something, make a copy of it from template_default, put that copy into the same folder structure under your new template, and make your changes there.
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