Build a one-page navigation within a non-one-page ttheme…

You can do exactly what you suggest. You do not need to create a separate menu or change the php of a page template.

If you add IDs like you suggested as anchors, then your menu items can just specifically target those IDs via the # sign.

To control the link to this degree, you will need to add “Custom links” instead of just selecting the page from the list.

Example Page: (domain.com/about-us/)

Item on example page : <h2 id="history">History</h2>

Menu item: Custom link pointed to domain.com/about-us/#history