On re-reading the OP, I think you're exactly right.
@Richard38213849xt3l, you are confusing setup of the application and UI with the setup of each document. They have (almost) nothing to do with each other. Styles and other document-related things are stored only in each document (with a sort of complicated exception we'll ignore). When you set up all the styles etc. for a document, they are saved in that document and completely "leave the building" when you close it. Opening a new document will present you with the very basic few styles that are integral to new documents, not any you created for another doc.
You can create documents and save them as templates, so that you open that template and have a new document with a starting layout, styles, settings etc. but that 's a very specific separate workflow and process, not the way things work for opening a new, blank doc.
I think you might be seeing ID in the way Word and other apps work, where styles and changes to styles are saved in a common template/file, so that each new document has access to those styles. With the exception of a few sophisticated processes, ID doesn't work that way. And none of this has anything to do with UI workspaces at all.
If you want to create a new document modeled on your prior EPUB source, open that document, delete everything that's specific to it (most content, I would assume, and any special/one-shot styles. Tidy it up, add any common new styles or starting content you want... and save it as an InDesign template. (Option in the Save-As menu; it will create an INDT file.)
Close it, then open that template file, and you will have a new, unsaved document containing exactly the styles and starting content you set.
Thanks very much! I called Adobe in my perplexity, and they said each of the points you say here. They showed me how to remove all pages but one from the original file where I created the styles and workplace, save it as a template, and use it for future new files. I completely misread the nature of workspaces, which have a lot less utility and heft than I supposed. Now I get it! And templates will have the same utility for me that I believed workspaces would have, just a different workflow. Thanks for your help!
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