I know how to subscribe to a community on a different instance. Is there a way to subscribe to the entire instance on Beehaw, other than individually subscribing to all the communities on it?
That is not a feature, you would have to subscribe to the communities individually - that said, please only subscribe to what you actually enjoy. It balloons needlessly costs when federation is used outside of genuine interest. It may also pollute the all feed.
It would be straight-forward to script I think (especially if your account is on the same instance). API calls to get a list of all an instance's communities, then loop through the list, and sub to each one. I guess you'd then want a background version, occasionally checking for something new.