I think like we said before, it depends on what you want to do with it. (Also Budget)
Sports/Action/Wildlife/4k video such as your son's surfing? D500. The focusing system on it is unmatched by anything short of the D5. This and the 4k video is what you're paying for.
Landscapes/still-life etc. Pentax would serve you fine, but you have to give up some of the AF functionality of the faster D500. Sensor stabilization and weather sealing.
Like we said before glass is 90% the answer. The Sigma 18-35 F1.8 is by every measure I have found, the best lens you can get for APS-C. The drawback is that it's not stabilized. With that in mind, the stabilized sensors of the pentax cameras pair nicely with it. I don't know much about tele options for APS-C, it seems like most of the good tele glass is intended for full frame (70-200 f2.8 and the like). T. Northrup recommends the Tamron 70-200 f2.8 with the D500. Tamron makes a 70-200 f2.8 for Pentax, but not sure if it's as sharp as the Nikon version as it appears to be a different lens (Check DXO Mark).
In terms of your current lenses, I have no experience with them to know whether they're sharp enough for a newer body to give you noticeably sharper images. Perhaps DXO Mark could help answer this for you.
If you could afford it, and could wait, a sensor stabilized version of the D500 (Assuming they ever make one) would be by far the best option.