Couple of things, first, get a brake fluid tester. Just because it's not old doesn't mean it's good, if a little water managed to enter it can ruin good fluid. It's a handy tool, and just keep the batteries fresh and even a cheap one would work well. Second, I think it's more a function of the track and driver, everything you have seems correct.

"RBF600 fluid, about 3 months old"
I'm pretty sure you aren't running into brake fade, but boiled your brake fluid. Most people I know who run Motul 600 swap fluid or at least bleed every event, else they boil. Like already said, a higher temp fluid would help a ton with this, and SRF lasts way longer without changing thanks to the higher wet boiling temps.

I tracked for a long time with leaky bleeder nipples and high temps. Absolutely destroyed the paint on my akebonos and got all over my wheels but keeping the fluid topped off, I was able to get by, as annoying as it was.

The Z1 rotors dissipate heat alright, but have less mass than others and this seems to lead to more frequent cracking.

FWIW, a lighter car with stickier tires will have less braking required and have less heat to dissipate as a result. Track matters a ton too. I run a full weight car with 200TW tires on brake heavy tracks and have attributed a lot of my own struggles with those circumstances.

I would try fresh fluid first since that's the easiest possible solution and good practice anyway (Motul fluid is not expensive).

But I'm inclined to believe that the Z1 2pc rotors are at least a factor of overheating if not the direct culprit. The lightweight Z1 rotors have much less mass, which means they cannot absorb as much heat. During repeated high performance operation they simply cannot dissipate the heat fast enough.

Seems like the cryo treated blank rotors are one of the better budget options out there. Although I'm very tempted to pick up one of DarkJak's brake setups - the price is right.

Everything i can find about Endless RF 650 is that its ******* god tier. Other than cost, is there any drawback to the stuff?

I was literally just watching videos about endless today. Apparently Mercedes uses it in their f1 cars.