The compilation failing still makes it sound like you have an issue with the kernel you’re compiling against, i.e. it sounds like you’re compiling an older driver version than you think you are (which is what would make the rcu patch necessary). After all, the kernel and nvidia source code we’re compiling cannot differ, unless you broke sha256 or your memory is faulty, or your configuration is not what you think it is. Make sure you update your hashes correctly, if you specify the wrong hash nix will just reuse the old driver sources.
If that’s still not it, maybe try installing from scratch to a clean partition with a really trivial config to reproduce it?
I don’t think it’s too surprising that you’re seeing known bugs with a (two?) year-old version of the driver not magically disappearing. Nvidia have been putting a lot of time into it since they’ve hit the AI marketing goldmine, and at least IME the modern driver actually finally works for normal desktop use, even if configuration is still messy on account of it being an out-of-tree module. They’ve also at least announced an attempt to merge it, at which point only the mesa incompatibility would be a problem. I wouldn’t exactly call this an act of newfound selfleseness, especially since GPUs from before ~2018 will remain broken forever, but at least the money is finally flowing such that we benefit from it a little.
Either way, this is a bit besides the point of this topic, we should move this part of the discussion elsewhere.