Conformal coating is ok but unless I spring for the expensive vapor deposited stuff then there won’t be 100% coverage. Particularly sharp corners or edges don’t coat well with typical conformal coating. And yes more coating would have to added after the hand soldering of the motor/DC cables. Otherwise, filling up the whole enclosure with potting is just easier.
Yes DIY kit means DIY whatever-proofing as well. Have it your way. That’s the point.
2 parallel PCBs is exactly what I’m working on unfortunately. Doing this helps a lot of other factors besides waterproofing… Give and take. Engineering is all about which pros and cons you want to deal with. You can never get rid of the cons. You can only just trade them out for different cons.