PicoMQ works with any S3-compatible object store. But I wasn't aware of GCS Rapid Bucket, it sounds a lot like AWS S3 Express, which is also zonal. And it does help with durability ACK latency quite a bit, keeping it closer to ~50ms.

I'll be setting up a GCP deployment example similar to AWS soon. I'll be sure to try Rapid Bucket as well, thanks for sharing!