I've used such a combo to thin saplings in Maine. The trimmer has a
straight shaft- don't even think about the ones with a coil-spring
"shaft." 3" diam., maybe 25' high are doable, with experience and care.

There are "flail-mowers" too, like used along various highways, that
use chains under a large rotary-mower deck. Really beat the crap out
of anything that gets in the way.

HTH,
J