I had the same quest, and tried to use bicycle brake cable, but which lasted about 4 seconds. Hard to believe, and which was potentially dangerous, as some other metal options would be. Maybe some type of fishing line would work.
Posting your question on a Sat. morning should get some interesting replies.
I could have waited but wanted to make a choice sooner. The weather here has been exceptional for growing, an answered prayer, but weeds come included.
FWIW I replaced the string on my trimmer a few years back with “Gator Blades” This was a set of 3 plastic/nylon blades about 4”(?) inches long..
If I was real careful I could get 2+ trimmings out of a set of blades. I could replace the blades in about 20 seconds.
The chief draw back for me was the 75 feet of chain link fence across the back yard. A little to close and you would break a blade.
Of course the chain link doe not like the string trimmer string either.
Regards
alfa6 ;>}