Solid approach for predictable key spaces — isolates the blast radius without touching the whole topic. The catch is when the hot key is unpredictable (bots, sudden high-volume users) — you need tooling to detect and re-route dynamically, which is a different problem.
Relaxing ordering on non-critical event types is usually the safer default at scale. But your suggestion is the right first move before you get there.
Jeongjin Shin
The oldest_unacked_message_age vs raw backlog distinction is the right one to lead with, most teams alert on the wrong metric early on. On the ordering-key hot spot problem at 10x, did you consider splitting the hot key onto its own subscription instead of sharding the whole topic? That way a bursty user doesn't stall throughput for everyone else sharing the same ordering key space.