I had similar complaints with string trimmers over the years. The
best result I ever had was to modify the head so I could insert
piano wire instead of the monofilament. I'm sure it was dangerous
as heck, but it worked and lasted quite a while before I'd have to
replace it. Unlike most uses, my string trimmer was used to trim
grass against a limestone wall, so there was terribly abrasive
conditions for monofilament. I have also used aircraft cable, but
the piano wire worked best.

If you try it, don't attempt to wind the wire onto the spool.
Instead, cut pieces of piano wire to an inch or so longer than the
recommended extended diameter of the monofilament, then heat it in
the center and wrap it around a bolt held in a vice to place a
loop in the middle. This then goes around the center bolt in the
string trimmer's head when you reassemble it. I've tried this
using just a single piece of piano wire and also two, placed at
right angles, giving me 4 strings instead of 2. I couldn't tell
any difference in lifetime or cutting.

Nonny

--
On most days,
it's just not worth
the effort of chewing
through the restraints..