I love Cali kings, I have 2.2 now to grow up and pair.

I do a lower hotspot as well, but still keep ambient temps around ~79-80 with a very slight night drop. They're hardy guys that enjoy cooler temps over warmer, so your temps sound fine. I wouldn't let them drop under 75 though.

I feed my little ones every 5 days until they get to be on adult mice or so then cut them back to every 7 days. I go by the 10-15% rule roughly too, or you can eyeball. Your snake looks like it can easily take mouse fuzzies/hoppers.

I personally don't give my kings bowls big enough to soak in, and I doubt you'll see your king using much of that "second story" ledge you have in there. It will probably be burrowed most of the time which is a good thing. If you feel the need to switch to paper towel substrate that would be fine too since that's what I use: they pee and poop a LOT so it's easier and more sanitary cleaning on my part.