Welcome ncee! PF is a friendly community even for non-Pentaxians too, so don't feel alien here.
Originally posted by ncee 85-95% Landscape photography.
For this use the K-1 will be absolutely excellent. How much of the last 5-15% is fast AF-tracking action photography? That action part is probably more suited for the Nikon D500. Fast tracking AF is as far as I know the only thing that D500 does better then K-1. That means, K-1 does have decent AF tracking too, but D500 is better. But for your 85-95% landscape photography and other non AF-tracking photography, the K-1 should be a better choice.
Originally posted by Na Horuk There is also the brand new DFA 15-30mm zoom, which according to reviews is pretty great. This should really cover the (ultra) wide angle range. Its not the most affordable, though. The Pentax DA 12-24mm does not work fully on FF (I think, feel free to double check)
If think OP referred to his current lenses on the D300s. So mathematically a FF 18-36mm lens should replace the 12-24 and a 27-450mm should replace the 18-300. The D-FA 15-30mm fits the wanted wide angle zoom range good. I hope Pentax will launch a D-FA 28-300 with HD coatings and WR within a couple of years.
For the super zoom there is no currently produced lens that replaces it well, but a
Tamron 28-300 f/3,5-6,3 may do a decent job for a very affordable price. Using the crop mode on 300mm should give the same reach as he is used to, only combined with a newer and better sensor.
If ncee likes 50mm on APS-C, the
FA 77mm f/1,8 should be an excellent replacement. There also exist a smaller and cheaper
DA 70mm f/2,4 that performs well on full frame from f/4.