What about bolting the ESC to an aluminum brick to be a heat spreader, and bonding the heat spreader to the inside of the cooler with epoxy? My fear would be that the fets themselves get too hot because the board has such little thermal mass under the low side switch drains.
Also, without test data, my opinions are just ravings on the internet. I could be totally wrong. I have engineering PTSD and so I am far to conservative sometimes, and I still miss things like @PowerGlider rationale for using non-conductive hardware. I wouldn’t have thought about the board getting wet but it is a possibility in the ocean.