It makes a difference if the 6" - 8" grade differential is spread over 8' or 28'. What dimensions of patio will you be creating? Why do you think the patio must be "perfectly flat"? Even a DG patio needs some pitch to drain.

The dimensions of the patio are 16' x 18'.

Could you create the patio as a landing such that at the high end the LAWN is retained by the metal edging and that at the low end the PATIO is retained by same (like it's a step)? That doubles the amount of grade differential you can accommodate with the edging. Combined with pitch of the patio itself (you did not answer that question but you must make drainage right ... or suffer) you should be able to make it work. In other words, at one end of the patio one steps down from the lawn to the patio. At the opposite end one steps down from the patio to the lawn.