The stars are going to track somewhat differently depending on the part of the sky you are shooting. Overall, in terms of the approach and process you are using, I don't think that I see anything you are doing "wrong". I have never really applied a 2 second delay, but that is just extra protection in terms of controlling the any vibrations.
I tend to shoot with a wider focal length. Last year, when I acquired the GPS unit was a year of learning. I tried all sorts of things. Based on that, I decided that I really did like the GPS unit, but to help things out, I did go out and get a very fast lens - the Sigma 18-35/f1.8. In some ways it's overkill, also it is a bit more difficult to use since it does not have a hard infinity stop, so focusing does take some time - and luck.
With the K5/K5IIs, I just start out at ISO 1600 and go up from there - with the largest aperture. I have found that at 18mm, 420 seconds (4 minutes) does start to pull star trails around the edges. So, as a note to myself - 240 seconds and kick the ISO up from 1600 to 3200.
At first I was shooting straight up at the sky, although I have not done that for a year or so now. I want to try that again. I also want to try shooting with a longer focal length. In a couple of the astro threads, folks have captured some really nice images at 200mm.
I have also been pretty successful at stitching shots together, without using the GPS.