I'd consider the Tamron 70-200 if cost is an issue or the DFA 70-200 gif it isn't.
But, the 50-135 is not soft edge or border in most configurations. You can go to a bigger lens, but getting better images is going to depend on technique. If the 50-135 is what you think you need go for it. It's a great lens. Only the best ZIess lenses are sharpest at ƒ2.8. Expect to pay between $2000 and $5000 for a prime if that's what you really want.
Look at this Zeiss at ƒ4. You pay a lot more for a bit of extra sharpness/
Zeiss Milvus 100mm f/2M ZE Lens for Canon EF - $1,843.00
Sorry, the two Zeiss lenses above ar not the same but, I couldn't get a lens chart for the Milvus. But it just gives you some idea of what you re looking at if you really want razor sharp edge to edge ƒ2.8. I would argue you can make do with a 50-135 and get excellent results. Your opinion of the 50-135 is not shared by any owners I know of. That being said, the 55-300 PLM might be worth a look. For shooting moving objects it's probably the fastest focussing lens on Pentax, but it doesn't go anywhere near ƒ 2.8.