Okay... Thank you all for your replies.
It looks like I really
need one lens, the Sigma 17-50 F2.8, which check all the boxes of my current needs : fast, quiet, sharp, versatile.
I can get one new in my country for less money than any other option (Sigma 17-70, Pentax DA 18-135, Pentax DA 16-85, any Limited). Only the Tamron 17-50 F2.8 is significantly cheaper than the Sigma, but it's a noisy lens and has front/back focus issue which depends on focal length, it's not acceptable for me since I only shoot with the OVF, which is sensitive to that kind of problem.
I also understand that I definitely don't need any Samyang. Thank you all for curing a part of LBA about that!
If I need a WR lens, the 18-50 is here. Bad weather do not happen often indoors, so a fast lens isn't needed! I know I don't like the 18-50 much, but I took quite a lot of good shot with it, I should not forget that and give the little lens some love! Thank you for saving that lens for the trashcan!
The Tamron 70-300 will go, one day or another. It is a fun lens, which taught me to deal with higher focal lenghts... something a superzoom like the 18-250 won't, since you can always zoom out. But when I got the PLM, the 70-300 had to retire, ashamed.
About the Tamron 18-250, I'm not ready yet to get rid of my only superzoom. I can see use of it still.. for now.
Finally, I understand that what I will
need later is the Pentax DA 18-135, for outdoor shots and hiking. For that usage, it will be better than the 18-50 (faster focusing) and Sigma 17-50 (WR, and lighter to carry), and more versatile than both of them. And when I will get that jewel... Tamron 18-250 and little 18-50 will likely be doomed.
I wish I could have added a Limited to the Kit though. They look fantastic... pixie dust and such. But so costly! And there is not much room for any of them because my kit is already almost complete!