As said a Bush blade or saw blade(did that one myself also, 7 1/4 inch circle saw blade) will do stuff up to 2inches thick,given you put it on a good weed eater.Make sure you get the kind with teethe like a saw blade! the 3 and 4 point blades vibrate to bad!
With that said what type of trimmer line are you using?
If its breaking or melting down inside the head.You might just need better line.Try to find Still ,or Huskavarna's Titanium line.It will not break off,of melt while cutting any kind of tuff grasses,small weeds! But obviously it will not cut saplings.
I have used a friends Field and tuff brush mower (i forget the name,the one thats on TV all the time!) Anyway the one he has is the older model that uses one big blade like a bush hog.It cuts anything that will go under the deck! Anything!

It is also self propelled.

If you could find one of these you'd be in business.Might try running a want add in your local paper.
