I have a slightly similar requirement (Federation as a way to "group" metrics and expose 1 interface instead of many) and was considering using federation for that, although I’ve also read that using federation to “read everything” is often a bad idea.
After seeing @stuart’s reply, searching about “prometheus agent mode” seems to show very interesting results and likely applicable to these scenarios. In case you’re interested: Introducing Prometheus Agent Mode, an Efficient and Cloud-Native Way for Metric Forwarding | Prometheus