Thanks for reaching out today.
You're right, for now you can only export a single mockup to JSON but I've added this request to our internal tracker so we can review and consider it for the future. If others would like to see this feature, please let us know (or add your Like/Heart vote above) to help us prioritize this request.
For the other options mentioned, you can actually export a single mockup or all of them to PNG, as well as export to a BMMLs zip file if needed. Here is the related documentation if needed: https://docs.balsamiq.com/desktop/exporting/
Hope this helps. Thanks for the feedback!