I will take a different view, based upon my own experience. I have had the DA18-250mm (my first ever Pentax lens), DA18-135mm WR and DA18-270mm. The DA18-250mm is a great lens, even by today's standards in my opinion. When I was looking for my first lens, I knew that I wanted an all-arounder. I went to a shop to try the Sigma 18-250mm and DA18-250mm. The Sigma was far too heavy and too big, for bushwalking. The DA18-250mm was perfect.
The DA18-135mm is a WR lens, small nice and very decent. My wife loves it. For me, it lacks the long focal length of the DA18-250mm.
I bought the DA18-270mm, because my DA18-250mm started to develop mechanical issues, after more than a decade of use (by me, and many more before with previous user(s)). The DA18-270mm is, like the DA18-250mm, the perfect all-around lens in my opinion. It is small, compact, relatively light weight and very versatile. I considered again alternatives like the
Sigma 18-250mm and Sigma 18-300mm. In each case, the weight and size was an issue for me. In addition, the Sigma 18-300mm was pricier than the DA18-270mm at the time of my purchase. If I compare side-by-side the DA18-250mm and DA18-270mm, I have been very impressed by the AF of the DA18-270mm, its speed, reasonable accuracy and it is almost silent. IMHO, the AF is a major improvement over the DA18-250mm..
Importantly all these lenses are all-around lenses. They cannot be compared with prime lenses in terms of sharpness and IQ: it would be like comparing oranges and apples. I am a prime lens shooter (see my profile), but I value highly the versatility of an all-arounder lens, especially my DA18-270mm.
In summary, the DA18-270mm is a very decent lens, excellent all-arounder, and perfect for bushwalking thanks to its relatively light/moderate weight and small size. I do not believe that the DA55-300mm can fulfill the same role.
I hope that the comments may help.