You’ll need to come up with some sort of custom solution for this kind of simulation.
Something akin to this:https://youtube.com/watch?v=iNC2X9r0oGY
But on larger scale.
With few millions of properly simulated actors you’re bound to hit memory limit and general slowness.
You need to treat them as some sort of particle systems where each actor is internally represented as something like a dot or doesn’t even exist, but is spawned as AActor on demand.
That’s pretty much the way to go.