"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.
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