There needs to be a feature in InDesign that will allow you to create a new document using selected pages within an existing document without having to delete the unwanted pages like you do when create a 'Save as' document. And also the ability to create from the existing master pages.
It's there already. At least after a fashion.
Create a new document of the same dimensions of the old one, with one blank page. Then, in the old document, select the pages you want, and from the Pages flyout menu, choose Move Pages. You can then select the new document to move the pages to, and make sure that Delete Pages after Moving is not selected.