NVM, found the answer! To anyone struggling with this:
To be able to use a template in Customize or have content that also updates to every page where it’s used when the template in question is being edited, you have to create the template in Theme Panel – My Library – Add New. And also edit it there. This is a separate location from Templates – Add New/ Saved Templates or My Templates.
You can find Theme Panel listed LAST in the Dashboard. (I could swear it wasn’t there half an hour ago…) And My Library can be found under Theme Panel.
