Just for the heck of it I would try reducing the WD Hitch up force by reducing the number of links bending the bars.
If adding the scooter reduces the tongue weight by 100 lbs. try taking away 100 lbs. of bar lift. It does not cost anything to try it and you know what it pulls like now.
Another issue is the fact that you have changed the center of gravity effecting the weight ratio with respect to the axels. What effect this has on the handling is unknown. Again maybe it can be offset some by reducing the WD hitch lift.
Personally, I am not sure my TT bumper or frame could hold 300 lbs.
Another option is to add a front hitch to your tow vehicle. That is what I did to keep my bicycles higher of the ground. The front hitch is rated for 500 lbs. tongue weight and 5000 lbs. of pull.
Good Luck and be safe.