All a Template file is in InDesign is a normal InDesign file that has a read-only attribute applied to it. You open up an INDT file and start working, and when you whack control+s or command+s to save, it brings up the Save As dialog. That's it.
When I want the styles in one document to appear in other documents, I go to the target documents and Load All Text Styles. It's kind of a manual version of style sync in Book files. You get a dialog where you can have all of the styles in your open document overwritten with the styles from the template, or you can pick and choose which ones come in.
I am really hoping that there is a way to more or less have multiple Document files reference a single Template file, so if I make updates to the Styles in the Template file the other files will update automatically.
This isn't generally the way that InDesign works - and the tools that do work this way are part of the Book feature, which you've already identified as unsuitable. It sounds to me like you are maybe waving the baton for a bunch of other designers or graphics producers or whatever, and you want them all to be forced to use the "approved" styles in a template. Maybe I'm wrong - in which case, we might be able to help you if you tell us more about what you're trying to do.
But when I have to wave the baton for a bunch of other people, I just say "Every day, before you start work, import the styles from JoelWavesTheBaton.indd." Said style-source document's only graphical content is a picture of me looking aggravated with the caption "Pink slips come to those who ignore style guide rules." (This mostly works because we all know that it's a joke. Your mileage may vary.)
Maybe a library would work for you.
This plugin from Woodwing is often mentioned when this question comes up.