Baby Step #1: Installation of 150' 600V underground cable

[BackwoodsEE]

Just jumping in, but I'm pretty sure he is running the DC+ and DC- from each of the four strings separately through the 150 ft run (using 8 conductors) and either combining them or running to independent mppt in the equipment shed.

Exactly. I'm combining pairs of strings in the equipment shed, with two charge controllers. The box at the array end is just a place to transition from PV wire pairs for each string to the 10x9 cable.

That ninth conductor works nicely as a backup EGC, with my metallic conduit (lock nuts on both interior and exterior of the boxes) serving as the other EGC.

[Sunking]

Just jumping in, but I'm pretty sure he is running the DC+ and DC- from each of the four strings separately through the 150 ft run (using 8 conductors) and either combining them or running to independent mppt in the equipment shed.

That is why I asked.