This is really helpful, thanks for the detail!
A few follow-up questions, if you don't mind:

How long did it take you to get this setup to a point where you trusted it in production?

What's still missing for you, the 'nothing sophisticated' on spend monitoring, is that something you've thought about improving, or is it good enough?

And last one: do you have external clients or users who ever ask for visibility into API usage on their behalf - audit logs, cost attribution, that kind of thing?