I have a rough idea of how I might test this idea out. Because I need to do this before the end of the year I'm keeping it simple. I'm going to build a single page app that guides a user through a specific protocol. This is free. I promote this free page.
When the user is finished with the protocol it prompts them to pay to get access to advanced features like saving, reminders, text analysis, and dozens of other templates and workbooks. It occurred to me as well while writing this that a workbook marketplace might be interesting aspect, especially if I can fill the buy side of the platform up with people who journal a lot. Though I'm a bit wary of marketplaces.
I also have a landing page if they want to learn more about the other templates and features.
If they click any of the "I want to pay for this" buttons anywhere it takes them to an email sign-up to get them on a waitlist. If I get enough interest then I build the platform and launch to the email list.