Your analysis on heat spreading suggests that it's possible to build a passively cooled solar powered mining rig on Earth too. Wouldn't that be strictly cheaper since you save the launch cost?
Or does the atmosphere add too much insulation?
I guess another concern is that on the surface you only get sunlight half of the day. Takes twice as long to earn back the investment. Though the equipment might last longer.
How well can 2 to 3 nanometer chips survive space? You'd definitely want a way to bypass individual chips, but that seems easy enough.