How do I get started with setting up a custom return-path domain?

As a practitioner primarily focused on observability and infrastructure reliability, I've recently been tasked with improving the transparency of our outbound email streams. Our current setup uses the default return-path domain provided by our ESP, which significantly limits our ability to isolate and diagnose deliverability issues at the infrastructure level. The opaque nature of aggregated bounce handling is a major pain point.

Therefore, I am initiating a project to implement a custom return-path (also known as a bounce domain or RFC 5321 `MAIL FROM` domain). My understanding is that this is a foundational step for advanced deliverability monitoring and sender reputation segmentation. However, the existing documentation from major cloud providers and ESPs often glosses over the intricate DNS and infrastructure configuration required for a production-ready, scalable setup.

I am seeking a meticulous, step-by-step breakdown from engineers who have implemented this at scale. My specific questions are:

* **DNS Configuration Rigor:** Beyond the basic SPF record for the return-path domain, how are you handling DKIM signing? Are you using a separate selector for the return-path domain, or is it acceptable to reuse the primary domain's selector? Please provide exact DNS record examples.
```dns
; Example for returnpath.example.com
returnpath.example.com. IN TXT "v=spf1 include:_spf.esp-provider.com ~all"
```
* **Infrastructure & Routing:** For a custom return-path domain, you must point its MX records to your ESP's bounce processing servers. What are the critical performance and redundancy considerations? Should the MX records point directly to the ESP, or is an intermediary MTA layer advisable for control?
* **Observability & Metrics:** Once operational, what are the key metrics and logs you expose from the bounce processing stream? How do you pipe Feedback Loop (FBL) complaints and aggregated bounce data into your central monitoring stack (e.g., Prometheus/Grafana, OpenTelemetry collectors) for correlation with campaign performance?

My goal is to architect this not just for compliance, but as a source of high-fidelity telemetry. I appreciate insights grounded in concrete benchmarks or data—for instance, any measurable impact on inbox placement rates post-implementation, or reductions in problem diagnosis time due to improved signal isolation.