SAM2 -“Health: Niddah isn’t bound by time. Zman Grama implies a set time, meaning a certain day or time of day. Niddah isn’t influenced by that. Rav Schachter has a whole Shiur on this. If I get bored sometime soon maybe I’ll try and find it on YUTorah for you. And like I said at the end, the Chiyuv would be there even when she’s a Niddah, there is just a separate Issur preventing it. That’s not considered Z’man G’rama.”
I could argue with both your Sevaros. But let’s say you’re correct -you could still say this is the Taam Hakra and it isn’t a Chok because the Torah only gave them Mitzvos that apply always.