re 4) If you're there, at the risk of drawing the ire of the "cloud is always too expensive" club, be sure you really really really want to run something like Kafka yourself, and not use a hyperscaler's platform queue/queue-ish system, aka SQS or pubsub or whatever Azure/your platform has.

Kafka has its own foibles and isn't a trivia set-it-and-forget it to run at scale.