I have the same issue. Of my 3 ponds, 2 have berms around them. I haven't figured out mowing those banks as my tractor is far too large. I have to undo years of minimal care and then figure out a plan.
The third pond is surrounded by grass. As the pond level lowers through the summer, tall grasses grow up and you can barely see the pond. I would say before me, a team of folks with weed whackers was how it was done. Perhaps this is how I will do this in the future, but for now since I don't live there full time, I'm using the same setup as the OP. I have a JD 4044r with a 72" rotary cutter.
I back the cutter out over the bank and lower the 3 point as far as it will go.
Not the best as it doesn't get down the bank, but at least levels the pond grass with the yard grass.
Sketchy backing that very large tractor that close to the bank lol
