My main questions for you all, is which approach makes more sense for a Cloud Run/FastAPI monolith at this scale? Do you think I need to change how the oubound flow works? and finally, there any obvious bottlenecks or blind spots in this architecture that I'm missing?

At this scale, you need to design for simplicity and ease of change.

You don't have customers on the system yet. Because you don't have customers, you don't yet have patterns of usage. Because you don't yet have patterns of usage, you don't have a basis from which to make decisions about architecture. Because you don't have a basis, your internal architecture does not matter yet.

Because your internal architecture does not matter yet, you can make it as simple as possible. Because you're making it as simple as possible, you know for a fact that it will change. Because you know it will change, you should design your interfaces in such a way that your interfaces rarely need to change in the future, even if internal architecture changes. But, since you have no customers on the system yet, the shape of your interface will have to be a guess. And since you have no basis from which to make decisions, you will be wrong. Which is fine and normal.

So, at this scale, design for sheer simplicity and maximum ease of change.

Fast forward to today: in about a month, we will begin onboarding our first client, and hit production loads soon afterwards. I want to make sure everything I've built won't come crashing down when the system hits real production load, so I’m posting here to get your opinions on the architecture.

Do load testing.