With what you have I would suggest sticking to the prime lenses, so the 15mm and 200mm lenses. The fact that you have dark skies close by is nice and will make up for a lot of othre issues like not having a tracking mount or not using astrotracer. I have gotten good results using an old screw mount Vivitar 135mm lens that doesn't have any ED or APO glass in it by taking a bunch of 1 second exposures and then stacking in Deep Sky Stacker. With those you should be able to get some good wide field shots as well as some good deep sky objects (DSOs).
For DSOs (mostly the Orion nebula/M42) I find that I can get pretty good results shooting with that 135mm lens at f/4, ISO6400, 1 second exposure. Experience has taught me that things work better if I recompose after about 2 minutes with that lens. Set the camera to 2 second mirror up delay to minimize the shake that the mirror introduces and always use the remote. Then it is just grab a bunch of shots (by a bunch I mean several hundred), stack, and post process. Also when stacking be sure to get some dark frames (lens cap on pictures shot at the same ISO and exposure time as your actual images) and bias frames (same iso as the other shots but at the fastest shutter speed) as those can really drive down noise and other artifacts in your shots when combined using Deep Sky Stacker.
Post processing is really the key here and is something that one needs to learn. I started off down the wrong path and thought I was making good progress and wanted a critique and got a education. Here is what I produced from that stack which doesn't look too bad but I quickly found out that my abilities in post processing were much lower than I thought they were:
Here is what someone who know what they are doing (Pete_XL) with post processing
was able to do with the same stack I created
For exposure times without a tracking mount I would suggest following the rule of 200 which is:
Exposure Time in seconds =200 / lens focal length
As this really cuts down on the chance of getting star trails
For focusing use live view and manually focus. I would highly suggest using a focusing aide, either a Y mask, bahtinov mask, or just something to create diffraction spikes.
If you want some idea of what you could capture (you would likely need a tracking mount to get something like this) there is this
post from Pete_XL where he used an old 200mm SMC M lens
Last week I took a class on astro photography and have some notes that I can hand off once I get home, as well as having some links on post processing and how to do it right (i'm still learning but am getting better). I will post what I can dredge up for that when I get home.
I recently got the O-GPS1 for my K-3 but as luck would have it every night that I could have gone out and shot has either been cloudy or stormy so I haven't been able to play with it yet beyond mastering getting it calibrated and shooting off my deck into the overly light polluted sky in my city where a star like Sirius is pretty dim. With astrotracer never go out near the suggested maximum exposure time as a number of people have reported issues but I would think you could get up to 20-30 seconds without much issue if you can get a good calibration, and do check the results for the first photo and if you have trails re-calibrate and try again until you get no trails and then begin your shooting session.
As always ask questions, post progress, and get some pointers on this topic over in the
astrophotography group. Pete_XL, VoiceOfReason, DrawsACircle and others there are really helpful. Hopefully you can get pointed in the right direction form the start and not wander down the wrong path like I did.