Grass ain't that tough!
Each spring, I run a bed definer around each of my flower beds, which total nearly an acre. Probably something close to 2000 linear feet of edging. I follow this with the string trimmer on the "mulch" side of that edging, blasting out all the little tufts of grass that have crossed the line from lawn into garden, and simultaneously breaking up and scattering all the edger waste into the garden, before scattering fresh mulch into the gardens. During the few weekends that takes, I will go thru a lot of trimmer line, reloading several times in just a few weeks.
But then I can go the rest of the season with reloading probably less than once per month, since I'm only cutting grass, weeds, and raspberry shoots with the stuff, not trees!