You can enable save either based on any event or time. In either case, the saved form will be visible to you in the form Portal page.
- Do you need an AEM account to retrieve a partially completed form submission?
Yes, you need to login to view your saved form. But you can enable autosave for the anonymous user as well.
- Do you need a forms portal to find those incomplete submissions?
yes, the form will be listed under Drafts & Submissions component.
Please refer below 2 documents for detail:
Adobe Experience Manager Help | Drafts and submissions component
Introduction to publishing forms on a portal
- Is this the only way to do this?
You can customize the save and draft component though but as of now, that's the only way.
Adobe Experience Manager Help | Customizing Draft and Submission data services