Oh baby, one month after transitioning the site from single-time payments to subscriptions, I've passed $10 MRR!
The site has consistently earned flat but steady revenue around $120 / month, so if that translates to subscriptions then I can expect MRR to grow by about $10 every month moving forwards. Not exactly a rocket ship, but definitely better than the once-and-done payment model that was there previously.
I used SaaS Pegasus's built in Stripe Checkout and Stripe billing portal support to manage subscriptions, so it was very quick and easy to add!