I just picked up a new trimmer yesterday for the pond. Got a Stihl FS250. Works great on the really tall stuff. Unfortunately it will still necessitate putting a blade on for some of the thicker brush. Even after the brush is dead from chemical exposure, it's so thick it'll wear down a wire trimmer really fast.
I can relate. I've got some kind of weed that when it dies it's as hard as wood and almost the diameter of a pencil. The weed wacker with a nylon cord is no match for it.
I bought a blade kit off a TV infomercial that is supposed to be adaptable to 99 percent of the weedwackers out there. Wouldn't you know it the weed wacker I have is the 1 percent that it won't work on?