Good idea Lowell.
I would like to add that you are comparing two very different focal lengths. The optical properties of a wide angle (21mm) and 50mm are very different.I would agree that 50mm renders very nicely... it's a classic focal length for a reason. But I think you are mistaking the magic of 50mm for the magic for the A series. I doubt that you would feel the same way about the 28mm 2.8, which has some of the "deficits" of a wider angle lens. Wide angle lenses bend light more, and introduce all sorts of interesting changes to rendering that you do not see in a more natural focal length like the 50mm.
Now if you want all metal and tiny, manual Pentax lenses are a good way to go... I just hope you realize WHY you like the 50mm over the 21mm... and I don't think it's DA vs. A.
Also, the advantage of "designed for digital" has to do with the coatings, rejecting flare better for digital sensors. May or may not be a real issue for you, but that is what the whole designed for digital thing is about. I'm not sure that changing to manual focus glass will help with the whole wide-angle vs, normal IQ issue.
Also, remember that tiny and aperture do not play well together. Larger apertures require larger lenses. The A 50 1.7 is nice and tiny, the 1.4 is larger, and the 1.2 is even larger. This is one of the reasons the limiteds are so damn tiny, they aren't super-fast. In my book, this is not really a big deal, thanks to ISO and the fact that these lenses tend to look good wide open. You can't get that performance from the 50mm 1.7 wide open... it usually goes to 2.8 anyways!
You might want to consider this lens as a compromise between 50mm and 21mm. I would personally go for the FA version but you want metal, so
https://www.pentaxforums.com/lensreviews/SMC-Pentax-A-35mm-F2-Lens.html
And there is always the 31mm LTD.