This certainly looks polished, but... What use case do you have for working on the token generation code so often that it warrants such a big layer of indirection?

I don't quite understand how all the fuss around different OAuth implementations is justified, especially not why you would need a separate library or SaaS for that. Implementing the client_credentials dance is like, get a token, cache token+refresh until expiration time, use until expired, exchange the refresh token, back to start. I mean yes, some providers want an extra parameter or have a strange token URL, but once you've grokked the general concept, it usually makes sense.