Garmin GT51M & deadrise single vs pair.

[Alicatt]

From what I read they don’t work as well on the transom because of the turbulent water, seems a bit fragile as well on the bracket

Bracket does look strong enough on them, assuming it is the same as the GT54UHD that I have - I think the GT56 is a little longer, plastic moulding on the transducer it's self and a folded metal bracket to screw onto the transom. It will be ok as long as you don't hit anything with it.
Mine is not fitted yet, waiting to get the boat out the water, so I can't comment on how it works while underway, but anyway my boat only does about 6kts at WOT