[usgroup]

Make a small cluster and test stuff on it ... like anything else.

Most of the scalability stuff is about failure modes and how your system responds to it; so you don’t need stress tests for that.

Know the properties of your system. Eg what carries load? How does it scale? How much can it bare? Then mess with it on your cluster. Etc.

I suggest you start with Kubernetes and try and put together something like a self scaling PostgreSQL Citus cluster . Put data in it and then smash at it with big queries when ready.