Point 3 is spot on. I think it would be a good strategy to avoid expire dates on subscriptions. Producers could take decisions on whether or not keep sending data by monitoring responses on the consumer's target URL.
Another thing that it should be worth mentioning is that some services batch notifications (e.g Facebook Messenger) so that they can send more data in a single POST request.
Another thing that it should be worth mentioning is that some services batch notifications (e.g Facebook Messenger) so that they can send more data in a single POST request.