I use a Sigma 150-500mm on my K-70 and, to my mind, it's a fairly top-line configuration. The problem with anything longer is getting the focus absolutely spot-on. The depth of field on a 500mm at f/8 is only a few feet at 100 yards, less at wider apertures or longer focal lengths.
Unless you've a much larger budget than mine, finding an autofocus lens much over 500mm will be difficult. Manual focus is finicky on a good day, even with "focus peaking", you'll probably be limited to using a tripod.
I carry a Samyang 800mm f/8 mirror in my bag as a last-chance effort for that really remote subject, but so often "digital zooming" from an image taken with the 500mm is as good or better than I manage with the 800mm. I'll even try a 2x converter on the 500mm on occasion but again, focussing the dim image can be very hit-and-miss unless it's a fine sunny day and the subject is in good light. OK for a "record" shot, but hardly worth the effort otherwise. Distortion from heat haze or shimmer, especially over water, can be a problem on a sunny day at any range, even more so at the sort of distances where a 500+ lens might seem useful.
Good luck