How can I ensure that I can retrieve my user's progress when they don't submit the form?

Using the new card form, with one question per page - with stripe as the last question for checkout.

If someone has completed all the fields and then don't complete a purchase, all their data is lost and I cant contact them to try and make the sale?

Is there a way around this?

That's correct - You won't get any info from your user unless they submit the form first. My recommendation is to enable the SAVE AND CONTINUE LATER feature of Card Forms so they can at least save their session.

However, this still relies on the user - Their session will only be saved if they opted to save it while filling out your form. Maybe you can add a note on your welcome page that they can save their progress by clicking the SAVE button at the bottom (on the progress bar).

Complete guide: Setting-up-the-Continue-Forms-Later-Feature-in-Card-Forms

This really kills it as an e-commerce tool. 60% of my customers book after we contact them back. 

Are there any plans to adapt it to save after each stage? Or any other workaround? 

I'm afraid the feature I just discussed earlier is the only way to save their session.

If you want this to become seamless, where there's no need for the user to proactively save their session, you can use a Classic Form (instead of a Card Form), then follow these guidelines on how to save and continue later using a session URL.

Related guide: How-to-Save-Forms-and-Continue-Later

In case you're not familiar on how to change your form layout, navigate to SETTINGS > FORM SETTINGS > SHOW MORE OPTIONS button > scroll all the way down to the bottom and toggle the FORM LAYOUT option to ALL QUESTIONS IN ONE PAGE.