I finally received the PCA9536 and assembled one of the pump controller boards. The board is mounted in the bottom area with the help of some 3D printed brackets. The board is connected to the 12V supply and the i2c interface that is routed in a slot to the controller. The frame used in this build didn't have this feature yet so I've hand milled it into the frame, the new design has this slot milled with the CNC making it less noticeable.
When I connected the controller I found out that the default pin state of the PCA9536 switches the MOSFET. I am currently using a workaround in the software, but this will be fixed in the next iteration of the hardware because it's currently possible that the pump would start pumping if the system crashes.
I am currently working on debugging the code and once that's finished I will start testing the automated growing feature and new light with some real microgreens, I will keep you updated.