Thank you for posting your question,
Based on your description, you can create an API pointing to the backend; this way, when calling for an access token, you can pass your Backend API identifier as the audience, and your access token will be valid JWT. This way, you can set a shorter expiration date for the token.
Here’s a quickstart for Golang as API → Auth0 Go API SDK Quickstarts: Add authorization to a Go API
If you don’t want to follow that and continue to use the opaque access token, you are not supposed to validate them. Instead, they are validated on the Auth0 server when being passed in the request for the /userinfo endpoint.
I hope this answers your questions.
Thanks
Dawid