At least with Postgres you can scale up trivially. Postgres will efficiently take advantage of as many cores as you give it. For scale out you will need to move to a purpose built queuing solution.
There are many options for scaling:
- vertically scale by adding more memory
- start redis instance on another port (takes 1mb) if decided to add more cores on the same vm
- separate data into another vm
- sharding comes out of the box, but that would be my last resort