Are there any specific reasons that you think Postgres won't be able to scale?

I don't have personal experience solving the problem you're describing but if you do choose to use something other than Postgres I think Apache Kafka might be interesting for some parts of your infrastructure at least.

Segment [0] has written quite a lot about their tech in a similar space.

[0] https://segment.com/blog/the-10m-engineering-problem/