My Ryobi 2 stroke 25cc, has a throttle.. There is no speed on the throttle that cannot cut grass.. This week over 2 days I did 3 and a big properties, front and back, without adding more line once.. Today I had to add a bit of line as the line just wore away. Didn't break. Just got shorter.
Thing is it is going so slow, it does not break when you hit a fence post, or rock. Grass still gets cut. You can always add a little more power if you need it, but just on grass and weeds, just try to find a speed on your line trimmer that does not cut it.
Less fuel, and less line.