Great read! Curios to learn if you have you considered TCO of such a hosted solution vs. a tool like Apache Pulsar? Has async requests, combines Firehose, Lambda functions on events and (if Presto is an option) even SQL all in one [1]. I am not affiliated with the Pulsar project, generally curious if a comparison was made by OP or others and how it might look like.
[1] https://corp.narvar.com/blog/why-we-moved-to-apache-pulsar