Hi there Ave!
Don here from Shopify!
There are definitely a couple of apps out there that can handle this sort of setup for you. :)
Check out Rewardify for sure as this one mentions on the app store page that it would work for subscription products.
As with any app in our store, you can reach out to the app developers using the email address or contact form with any support queries you might have. :)
We're also available 24/7 via our support site for any Shopify-related questions you have!
All the best. :)