Something like this might work for you? I found it searching for "string weeder attachment to keep it 2" from ground". It's probably brand specific, but for that big of an area, I'd probably get a new string trimmer that would accommodate it. I can fully understand needing something to make that job easier, just trimming my edges with a lightweight one my arms get tired and it invariably slips to gouge out spots.
https://www.amazon.com/dp/B00HH4K6TC/
