Here is what we would do.
I've got an old articulating hedge trimmer head that I use for brush purposed.
Basically put it on the end of the string trimmer, set it level with the ground and use like a scythe or sickle.
No flying debris, just lays it over, and you can get through saplings with it. Stihl also makes a specific scythe for this type of job, it's the same as the hedge trimmer, only half the length.
Charge your time in doing it and call it a day.