Clip Studio Paint actually has a rain-fill tool. I was disgusted with it at first, because it only made straight up and down rain, then I realized I could just rasterize the layer and tilt it myself. I use a light gray color then change the layer to 50% opacity. Super easy.

Also, there is a "droplet" brush in the air brush tools that I used to sprinkle them with. That is also on the "rain" layer, so same color and opacity.