Ah, thank you for telling me about the AVX and also linking that issue. It is definitely the same issue. I read this comment on that thread (comment), which confirms that the very old intel gpus (like mine) do require the intel driver like Ive been finding out.
But yes, I installed Arch linux on the computer in question this morning to test and i was able to confirm that the xf86-video-intel driver does still work on arch with the exact same hardware. Mesa, the X server, and most of the other relevant software is either the same version or slightly newer than NixOS, so it doesnt seem like an upstream regression in any of those packages. Rather, it seems like there is some issue in NixOS that is causing this (source).
Hopefully this issue is able to be resolved, I am still investigating but I am nearly at my wits end with my limited knowledge of the inner-workings of NixOS and X.
Mesa version on arch testing system: 24.3.3-2
Intel DDX: 2.99.917+923+gb74b67f0-2
The computer also had Debian 12 installed on it prior to wiping it and installing NixOS, and X was working fine on there too. I am using NixOS stable 24.11