So I decided to texture the individual bricks inside zBrush. That way, I will be able to export a finished tiled texture in the end and also have the finished and polypainted brick meshes in case I want to do something entirely different with them.
I started by jumping into Substance Designer and creating a base graph for the color of all the bricks. It is basically a few noises with gradient maps for the plain texture. Additionally, I added more details like black / colored spots and dust traces which can be turned on and off. Having set up this base graph and exposing all the important parameters, made it pretty easy and quick to generate four different versions of my texture.
I switched back into zBrush, gave all the bricks a basic UV Layout (UV Master Unwrap) and applied one of the plain textures to it. I then started using all the other texture variations I created and added detail using the Projection Master and the Alpha Brush tool.