If I were to acquire a K-1, in my way of thinking and shooting, I would not want to be without an APS-C counterpart, even tough the K-1 can switch to APS-C crop mode, which is a wonderful convenience negating the need to always carry two cameras. Each has its own advantages and disadvantages, one issue being weight and size. But this depends on what shooting interests one has. If the main interest is in the wide to mid-tele realm, such as a combo of landscape, portrait, architecture, and the like, and portability is not involved in such as a lot of hiking or bicycling, the K-1 will certainly meet those needs very well. When going to location by car, weight can be a secondary consideration.
As to resolution, there is at least as much to be had with a 24mp APS-C model, which has great pixel density. The K-70, and especially the KP, have definitely narrowed the gap in terms of low noise and high resolution retention at higher ISO levels. I am curious as to the "many disadvantages" the KP or the K-3 II might have compared to the K-5 IIs. The K-5 IIs has been shown to have slightly better dynamic range than the K-3 series, as well as slightly better higher ISO performance. But neither is true of the KP. If you do burst shooting of fast-action subject matter, you might be best off to patiently look for that lightly-used K-5 IIs, since the K-1 is not exactly a monster for such use either. I do not blame you one bit for liking the K-5 IIs so much. It is still a very fine camera. From what I've seen, I like its controls better than those of the K-3 series. There is, however, a forthcoming K-3 II replacement in the offing, which might come pretty soon.
As to the K-1, the Pentax selection of FF lenses include some very fine ones, whether current designs or legacy. In some cases, FF is the only way to go to make maximum use of certain lenses. The FA 35mm f/2, the DFA 28-105mm f/3.5-5.6, as well as the FA 43mm f/1.9 come to mind. There are no APS-C counterparts to these two prime lenses.
OTOH, there are certain APS-C lenses that are also outstanding. The DA 12-24mm f/4 is a wonderful wide angle zoom lens with great edge-to-edge performance. The DA* 50-135mm f/2.8 is an outstanding tele zoom lens for APS-C, and amazingly compact for what it can do. It delivers images comparable to having a 70-200mm f/2.8 lens on a FF body, and the DA* 200mm f/2.8 (which is really a FF lens) is similar to a 300mm f/2.8 on FF, at a fraction of the size and weight in both cases, and at much lower cost. The DA Limited series of primes are also little jewels of excellence.
Last edited by mikesbike; 12-10-2018 at 12:30 PM.