I have two K-S2's and a K-70. For me there's not a
huge image quality difference in my day-to-day shots. The added pixels give me a bit more crop room which is appreciated, and the dynamic range seems improved as is the overall reduction in noise. The heavy-cropping pixel-peeper in me says thank you.
The K-S2 already handled low-light surprisingly well, but there's absolutely even better results with the K-70. That's been one of the biggest surprises for me. I've no qualms with 6400iso at the upper range. Overall too it just feels quicker. The menu structure is better, as is the grip (and very noticeably so), feeling much more secure with the heavier lenses. I have far fewer inadvertent 4-way rocker touches too, so I think the placement of it has been adjusted a bit. That's something that's irritated me on my KS.
So I think the K-70 is a very nice improvement on the sometimes underappreciated K-S2. It keeps most of the good stuff from the camera you have now, sprinkles a couple of flagship features on top (even some that the K3II doesn't have), yet manages to keep it simple and straightforward.
I had considered both the KP and K3II but now that I have the K-70 I don't feel I'm missing out on anything from those bodies. TBH there's really nothing I need that it lacks.
Just do it. It worth it. But keep your KS-2...