Is there any way to mutate the content of the response (e.g. page) in the middleware?
I should also mention that I don't want to push a new route (change the URL).
Mutating the content of a page in a middleware (in some Express-y way I guess) while staying on the same route is not something that you usually do on the front-end. Never saw it and it's a pretty good anti-pattern IMO.
You can make ugly conditionals by wrapping your whole template tho.
Best way would be to receive a different payload from the backend and adapt to it in your front-end.