First of all, don't feel silly at all looking at top-quality lenses as a beginner, especially if IQ is really important to you.
Many of us here have been through a multitude of lenses, looking for the keepers. Actually, I think most of us are trying to find the cheapest lenses that are satisfying to us. And there are a few "sleepers." But for the most part, you get what you pay for, especially with zooms. It is far better, and cheaper in the long run, to buy quality stuff right off the bat than to get on the lens buying/selling merry-go-round.
Secondly, I assure you that everything you need is available for Pentax. Going with either of the big boys may only "muddy the waters" for you. Your only problem is going to be figuring out just what it is that you need. Folks here on the forum can help guide you, but, ultimately, only you can decide.
Now, on to your situation specifically. I would not want to tell you to cancel your 12-24 order, because you may turn out to love that lens. It has a lot of fans here. On the other hand, I would encourage the 16-50 thought. To me, it sounds like it would be perfect for the majority of your shooting. And as you already know, it has wonderful IQ. The 12-24 will have a little better IQ, however, in the over-lapping area, as the 16-50 is weakest at its widest. Whether this is enough to justify keeping the 12-24 is one of those questions that only you can answer, as is do you shoot enough between 12 and 16. The 16-50 will likely also be able to do your "detail" shots, as it's minimum focusing distance is only 1'.
As far as an ultra-wide prime is concerned, it's pretty slim pickens here, I'm afraid, and options for Canon or Nikon would be
uber-expensive. There is a Samyang 14 that is manual-focus only, and some fish-eyes.
If you turn out to really need something like this, though, you should try the DA15 Limited. It is an absolute sweetheart, and the only wide-angle I have any real desire to use (I had a Sigma 10-20 that I just didn't click with). I would wait on this, though, and see how much you really need it. There is a "love-letter" thread here on the forum dedicated to this lens.
Anyway, the 16-50 is a great place to start. From there, you just have to ask yourself "what needs do I have that this lens is not meeting?" If you're really lucky, the answer will be "none."