: make sure the back-stay block is really sturdy: fit a Dyneema
backstay which is split quite high up, shackle each leg to the outboard
end of the davits: fit an "X" of S/S between the inner and outer ends of the davits arms, bolted rigid: wind
up the backstay as tight as you can?
You could even fit struts from underneath, bolted to the transom at the lower end and to sturdy plates fitted onto the dinghy
(watching out for any foam between the dinghy skins), with bottle-screws which can be turned to push upwards.
I saw a boat
which had telescopic arms which extended horizontally aft from the upper bars of the pushpit, but I don't think they could be made strong enough for the OP!
Time to buy an alumin(i)um RIB