After being live for almost 5 months I finally put up paid plans. With patio11's words ringing in my ears I opted for $7 and $9 tiers (more than I originally planned) with 20% discount for annual.
This also necessitates removing the app from the Github Marketplace (note this is a manual process, it will be live until they process the request). Unfortunately GH does not allow listing paid plans until your app is verified, which requires 100 installations. From my early users it quickly became clear that the server costs for supporting 100 installations far exceed what I'm willing to pay before getting paying customers... so I rolled my own subscription plans instead.
So, the hook is in the water - now I just need to get someone to bite 😅