Don’t build anything right now. Put an option somewhere that makes sense that says “like this? Upgrade to get even more features” or something similar. You know the users/audience better than I.

And put some sort of analytics on that button you can track. Once someone clicks it do a simple alert(“we don’t have a pro plan at this point but thanks for showing your interest. We will follow up later when we do”).

Then, let it sit for a week. IF you get some non 0 amount of clicks, reach out to the people who clicked it and ask what they’d like.

That should take you less than an hour to implement and be a great first step.