Curious: Are there any OSS entitlement management systems or libraries out there?
Unleash is an amazing open source feature management tool:
It would probably take some effort to hook it up to whatever billing platform you use, but it would make it easy to turn certain features on/off based on billing. Instead of checking if a customer has a specific plan, you instead have Unleash handle the features that are on/off and then check for those flags in the code.
Source code at https://github.com/Unleash/unleash
Setup instructions and SDKs for a number of languages in the README.