The unfortunate truth is that saplings require a LOT more guts to a trimmer/brushcutter than is required for grass and soft vines.
The other unfortunate truth is that cut-off saplings come back in multiple, with an ever-increasing root system.
The saplings need to be pulled by the roots. That is it for them. Then you can mow your field or paths with the extended hedge pruner, much faster than a circular blade because you're taking a 24" wide swath, and with the hedge pruner head two-sided, you can cut that wide swath either left or right.
7 or 8 years ago I bought one of the last Husqvarna combo boxes, a 32 cc power head with straight-shaft and interchangable heads, almost identical the Vangellis' picture. See the picture for where the interchangable shafts interchange. Mine had a string trimmer and interchangable blade that would change over at that same place, and an angled hedge pruner head, and a sidewalk edger that each will attach where the red arrow is.
I have never used the edger, used to use the string trimmer but don't anymore, quit using the blade because it was so slow compared to the hedge pruner and the hedge pruner itself?, that gets used every now and then when it suits the job. Large swaths of tall grass is where it really kicks ass. And hedges and bushes of course.
It would serve a landscape guy much better than it does me. It could be a very money-making tool but see, I climb and it doesn't. The whole kit came in a reinforced, wooden, lockable tote box. One of my most-favorite, least used power tools. I like the versatility of it.
It's a multi-purpose, commercial-quality tool and should last many years. Vangellis has almost exactly what you need, but the hedge pruner attachment is a MUST for what you want to do. Those three options should cover you completely. You should make it go out and pay for itself over a couple days so you don't have to moan the price.
The saplings are kinda fun to yank out by the roots if you have the right tool. Its kinda testosteroney, and once they're out by the roots, they're out. Its over.
You'd be surprised how fast you can uproot a hundred saplings (less than wrist-diameter).
How much area are you talking about, an acre? That could be 50 saplings or 50,000.