We have a few different backend services; I'd prefer not to implement SuperTokens in each one. Is there a best practice on how to manage authn in a such a multi-service environment? E.g. should our user service mint short-lived JWTs, once identity has been established?

using JWTs in this case is the right thing to do. The only con is that you will be exposing the JWT on the frontend storage so little less secure. But that's OK since these JWTs are short lived anyway