@don_park
DA15
It would help if you provided a few samples of the softness problem. DA15 is a complex design, being so small and not all are great sample. It may be you got a dud. If you post say 1-2 web size shots with center/border crops at say f/8 that would help understand the issue. We would be able to say to you if the lens a lemon and you should get it serviced or exchanged or if you ask too much.
I can also send you 1 or 2 raws files of your choice: choose 1 picture or 2 on my flick and I'll upload them so you could see for yourself what it is really about. I say that because yes I post process my files, I typically don't touch sharpness myself, but I think DxO normalize it automatically from lens profiles. And sure that depending of what I want to shoot, I add some contrast/micro contrast.
F135 and 135mm lenses
The F135 f/2.8 is an old Pentax prime. You can buy it used, it has AF (very fast one). The lens is able to give very interresting bokeh and portraiture shots. It has purple fringing, in particular wide open. But for an all metal FF lens it is light and small (395g, 68x80mm) taking 52mm filters. Like some ltds, it has the hood included directly inside the lens and you can extract it when needed.
I had the 50-135 before, it make great photos, has far less purple fringing and sharper too. But for me the F135 is sharp enough, and I really prefer overall to get FA77 + F135, have something still smaller/lighter, don't have to take both all of the time and get something far less intrusive. I think the F135 is also better for portraits. The alternative for me if you really need reach would be the 55-300 f/4-5.8. I just got one for my birthday, as I plan to use it for a safari in Tanzania next september. In a few month hopefully I'll be able to provide some comparison.
Many people like the F135, and you can find it used or the same formula, similar performance, I think colapsible hood too in its FA form: FA135 f/2.8. I didn't find it difficult to find. Today for example, there one FA sample for 290$ in Korea from a user with good rep on ebay.
Here the link to the specs of review of both 135mm lens:
F135 f/2.8:
SMC Pentax-F 135mm F2.8 [IF] Reviews - F Prime Lenses - Pentax Lens Reviews & Lens Database
FA135 f/2.8:
SMC Pentax-FA 135mm F2.8 [IF] Reviews - FA Prime Lenses - Pentax Lens Reviews & Lens Database