I can only tell you what I did with the plate. I used stainless steel sheet metal screws long enough to go thru the plate and into the transom 1 inch. I applied Marine Grade Goop into and around those transom holes just before I attached the plate.
3M5200 is an excellent sealer but I don't think it is designed to be an adhesive. I would never hang a transducer off the transom without screwing it onto it some how.