How do I archive all the completed Forms from my last project - and then using the same Form master, start a new round of Form collection ?

To disable forms, select a form in the builder and click the More drop-down. From there click on disable and the form will no longer be active.

To backup/archive form submissions, you can download them as an Excel/CSV/PDF. Refer to this guide: https://www.jotform.com/help/73-How-to-Download-Form-Submissions-as-Excel-CSV-PDF

To delete form submissions after backing them up, click delete all button on the submissions page. Refer to this guide: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data

You can also download all your data at once by visiting your account data page: https://www.jotform.com/myaccount/data