Also, is there a feature comparison between Hydra and, say, https://github.com/go-oauth2/oauth2 ?

The project you linked is just a library. If you use it, you must, in essence, implement the subset of Hydra functionality that you require. Hydra is a server that you can deploy without writing any code. The library you linked would be comparable to Ory Fosite[1], which Hydra uses as a dependency. [1]: https://github.com/ory/fosite