I think if you can't get a decent shave on 6 you have some technique issues unrelated to the razor.It is possible to increase the GAP to more than 18. That depends on the length of the screw.
I have not tried .. But maybe you can.
I'm fine on N (aka 3) myself.I don't usually move from position 6.
But I will see what happens in 7 or 8.