> The official solution is to fan out to multiple streams

Not anymore. Enhanced fanout allows up to 20 consumers, doesn't count against read limits and has lower latency than polling.

> It's very, very easy for provisioned capacity to be more expensive than on-demand.

Reserved capacity will beat on-demand pretty handily, even with massive overprovisioning. Assuming a 50% target and a 16 hour daily duty cycle, reserved provisioned is 20% the cost of running on-demand.