Thank you for your comments and reassuring me that the 60-250 might be better off in terms of auto focus (SDM) issues.
I just found a new alternative after reading this comment:
"Adam Nov 6th, 2017 2:03PM
We did some internal testing on the 50-135mm and found it to not fully cover FF on its own, but if you pair it with the 1.4x teleconverter, then it does, in fact
So you get a decent 75-200mm F4 out of it.
Read more at: https://www.pentaxforums.com/articles/photo-articles/how-to-modify-da-star-60-250mm-f4-full-frame.html#ixzz55Nt8voYw"
Which of those two options would you prefer?
Pentax 60-250mm or Pentax 50-135mm + teleconverter?
Will the AF still fully work when using the teleconverter? Will the optical quality be affected by the teleconverter?