I do know a fair bit about ESC design (all my robots use homemade ESCs. And yes, I will agree with you on the lots of trial and error front!). I wouldn't expect those FETs to handle 500A in real conditions, but I'm not seeing where you're getting quite such low current figures from (though those FETs don't have the clearest of datasheets)..

I do agree though it is generally better to use many smaller FETs than one big one; my first HW ESC used 32 x 100A FETs ('400A' per channel), which worked nicely until it caught fire due to a driver fault.. Making ESCs that work is easy once you know how... making ones that keep working seems to be the real trick.. The latest version is holding up well so far at least :L