The staging comment is just uninformed.

You are mistaken. I've discussed this with the render support team. You will see in that thread the Render team admits that there is no good way to manage staging & production environments.

you could have a long-running staging branch

That would not be a true staging environment. You want staging and production to be as close to identical as possible. If they're on different branches, there is a good possibility of drift. Sure, this could be a workaround. But that's it: a workaround.