I'm not a member of the "you have to tow anything with a huge SUV or pickup" school, but I do think a good guideline is that the load is 50% of the tow vehicle weight, 70% at the most.
Towing 5600 lbs with a 3500 lb vehicle seems a bit much to me.
This is baloney! I've towed upwards of 2x the gvwr of vehicles without issues. THAT is about the max I would do. half the wt, would put my old dually crew cab 4x with a 300+hp/600=lb torque rig at maybe 4000 lbs of trailer!?!?!?!?!?!
If you want a max ratio, the one I learned back the the early 80s before rigs had gcwr's, was 2x the grawr for the max trailer wt. So a sueby has probably a 3K ra, so a 6K trailer would be about max for the chassis.
Now HP and torque.......those become a guessing game as to how much you really need per say. Europe rates sueby's at upwards of 5K lbs, here in the US you might get 3000-3500 lbs with the SAME rig.
THings that help in towing, a rig with plenty of payload, hp is secondary! a rectangular wheel base is prefered over a squarish one. Ie cj5, big blazer/bronco have squarish WB's. My dually crew cab, VERY rectangular! While I said HP is secondary, to a degree, more is better. BUT, from a getting going standpoint, torque and overall lower gears is what you need on a steep boat ramp. So a 4lo option with a smaller HP motor may be in the end a better option than a rwd BIG motor per say, as it may not have the gears to multiply the torque so you go up the hill!