Thanks! We stuck with plain EC2. RDS has a limit of 80,000 provisioned IOPS and our read replicas on Postgres 9.6 would regularly hit near double that during peak
Did you consider lowering those IOPS with application-level and/or distributed in-memory cache and/or pub-sub notifications to let your app nodes not pester the database so much? Reasonably performant hand-written SQL (no ORM!), review of query plans, maybe shift the hot path into functions/procs?
That's an impressive amount of IOPS. I have always been a EBS / pd-ssd guy, and mostly rely on memory to reduce the IOPS requirement. But as cloud providers typically charge a ridiculous amount of money for memory, a setup like yours with instance storage / local-ssd is an intriguing option.
Apparently I'm living in the twilight zone because I have a vivid memory of reading the Aurora docs and seeing the same limit. Oh well, it's something to consider for the next upgrade.