Good point on the amplification of the front wheel movement. In my case I have a pretty smooth rocked road all the way around the pond. I just traded small tractor this spring and the new one has a remote outlet so I can use an external cylinder if needed.
Here are a couple of other options, if money is little object.
Tractor mounted DR trimmer ATV tow behind trimmer The problem I see with both of them is they are made for flat ground. No provision for tilting the cutter head to match the slope of the bank.
We have three 15' flex wing mowers for bigger tractors, but it is a bit of overkill and a person has to try and control the wing laying over the bank with the wing cylinders.
I like the idea of the tractor mounted one. It does have provisions to angle the cutter downward.
"The pivoting deck is ideal for roadsides, pond banks, and any other areas with uneven ground. It allows the trimmer head to follow the contours of the turf, leaving it trimmed and tidy. "
Take a look at the video and it shows it.
The only problem with it is it doesnt reach out very far. If your tractor is heavy, banks are wet and/or steep, your tractor may take a dip trying to get to the water line.
