DA 18-135 not sharp allover the frame ?
I think that you have to look to the strong side of the lens. When you need sharpness allover the frame , mostly with landsscape, than the lens is normally not wide open to get enough DOF. When I look at the numbers of Photozone than the 18-135 scores imo better than the DA 21 witch is a lot more expensive.
Compared with the DA 21
In my view it is a good thing to look at the strong points and that is a lot, much more than I expected.