heavier tongue weight mitigates sway .
in the extreme case visualize the axles near the very front of the trailer . vizualize the sway .
in the other case visualize the axles near the very end of your trailer. vizualize no sway .
i have nice shorelander boat trailer .
i moved the axles back about 16 inches .
i tow with my hd 2500 pickup.
i have no sway in my boat trailer at 80 mph headed for the delta .
i also replaced my factory 13 inch wheels and tires with 14 inch tires and wheels for an additional 50 % or so load capacity .
i raised my trailer fenders 1.5" to do this .
i frequently tow my boat very long distances at night , going and returning .
i have not carried a spare for my boat trailer for the last decade .
the OP should be very glad his axles are more rearward .
i have never had a flat or a wheel bearing failure .
i have so much reserve capacity that there is no threat of tire failure .
i am the guy in your spot on the water because your boat trailer had tire issues .