Do I have to finish my menu before I add the POS?

No, but it helps. The POS sells from the same menu your ordering page uses, so anything you add later shows up on the counter by itself. Most restaurants get their categories, prices and options in first, because that is the slow part, then pair a device and spend ten minutes adding people and their four numbers. If your menu is already in a spreadsheet you can upload the whole thing at once from the Menu screen. Getting started in the docs has the order we suggest.