I think you might mean the Fuji XT-3, which is listed as their current top mirrorless APS-C model. The K-1, as has been said, is a completely different animal. It is not of APS-C format, but the larger full-frame sensor. Your image size in the frame using a lens of the same focal length will not be the same. The full frame (FF) sensor will present a wider field of view (VOF). It also has an optical VF instead of an electronic one, so it is not mirrorless. The camera will also of necessity be larger and heavier. There are some FF mirrorless designs, the most prominent, I believe, are by Sony. Many people own cameras of both formats due to each having certain advantages.
Fuji specializes in mirrorless APS-C designs, while Pentax specializes in APS-C models of DSLR, which means having an optical viewfinder as apposed to an electronic one. Again, each has its own advantages. With an EVF, you get an electronic representation of reality, while with an OVF you are looking through the lens directly at reality. The EVF can be electronically manipulated, including illumination, or zooming, since it is a representation. With the OVF, it is what it is, but it is reality. Because of the OVF assembly, APS-C DSLR cameras are generally larger and heavier somewhat, compared to mirrorless designs.
As far as changing to Pentax as your over-all system, much would depend on whether you have specific shooting interests and what those might be. For instance, if you are much into sports or other action shooting, and do a lot of burst shooting, and continuous AF, the Fuji XT-3 has outstanding specs for that. Its video AF performance will no doubt be better.
But you bring up a more retro design, which of course includes controls. My observation of the XT-3 is its controls are indeed retro, reminiscent of manual-focus 35mm film bodies of the 1970's. I see the shutter speed knob on top, with also an ISO setting knob, and one for exposure comp. Very straight forward, as it was then. But there are disadvantages too. I see the setting jumps are in full stops, while my Pentax DSLR bodies can be set for 1/2 or 1/3 stop jumps. Considerably more sophistication.
The APS-C DSLR closest to being similar size and retro look as the XT-3, offered by Pentax, is the KP. It of course has its OVF. Its VF spec states a magnification of .95x which is better than the .75x of the XT-3. It also has a built-in flash, handy for immediate needs, such as daylight fill flash. Yet it is very close to the same size and weight. About the same dimensions, and 1.5lb. with card and battery vs 1.2lb. for the XT-3. Just as easy to set shutter speed with the KP via the front finger dial, and shown both in the VF and on the rear screen. Actually, with the exclusive Pentax Hyper System, you can instantly change either shutter speed or aperture, using the thumb or finger dials even when in Program mode, which instantly shifts the camera to aperture or shutter priority, and returns to full Program operation with a touch of a button. The KP's controls are exceptionally efficient and offer enormous options. No touch-screen though. Many of us do not prefer such, and with the KP's fine controls it is not needed. As to the weight, depending on lenses, sometimes the Pentax KP DSLR ensemble is equal or lighter than the Fuji XT-3 ensemble. The excellent Pentax DA* 50-135mm f/2.8 tele zoom lens on the KP is at least as light as a Fuji's similar lens on the XT-3. The KP also features in-body image stabilization, which will be in effect with any lens, even very good vintage lenses, some of which can be had at low cost.
So again, much depends on personal shooting needs.