I am wondering if you gave a larger bridge some consideration when you broke it out and what factors swayed your decision to stay with the 8A bridge. My experience with MOSFET PWM drivers is limited but my impression is they are fussier than triacs . j
I think keeping it cool is the key difference, I not only bolted it to the chassis, I did actually use real heatsink paste to help with the heat transfer. I happened to have the 8A bridges on hand, that's why I used them. Like I said, I initially thought I'd just unsolder the existing bridge and use it, but when it resisted removal I decided that it wasn't worth the risk of wrecking the board to try to save it.
I agree about the FET vs triac, I suspect the FET's are touchier as a rule. OTOH, I use them in a host of switching applications and have had minimal issues with them dying.
I also suspect that it's only in extreme cases that the rectifier gets hot enough to be an issue, normally a four motored A-A would not be drawing more than a couple amps at most under load.