I do this exact thing on my main site Productivity Guild. Any topic in the list with a shield in front of the title is behind a paywall. I’m doing this with this plugin:
https://github.com/discourse-league/dl-teaser
It lets you pick the icon and the location of the redirect so you can set up a topic or page with instructions for paying.
I’d be happy to tweak this to your liking given your budget for this. But I thought I’d throw it out there as an option.