You are 100% right about thr wiki, and it doesnt help that there's an unofficial wiki that pops up in search results too. And the wiki it especially out of date and confusing for nvidia. Its a known problem, and a bunch of "high level" contributers complain about it in discorse threads all the time.
You linked "nixos-hardware" repo, so you've obviously done your homework. If youre not importing that as a flake, and importing the "common-gpu-nvidia" (or whatever the module is actually exposed as, thats just the pathing) modules, and instead grabbing the values manually, then you also need to grab the ones from here: https://github.com/NixOS/nixos-hardware/blob/master/common/gpu/nvidia/default.nix
Your gpu is too old to be includes in the cuttoff that nvidia did when starting to open source their drivers, but i wouldn't say its too old to be usable or anything.
The wiki is out of date when they say power Management is experimental and unstable, I recommend setting that to true.
I think the main problem is using nouveau drivers, and setting the option provided in the link above should switch that to their closed source drivers. If you make that change, definitely do a rebuild boot instead of switch, and then reboot into the new Gen.
Its so annoying thag theres open (nouveau), open (nvidia - but only newer gpus), and closed (nvidia). There's been a lot of talk about making major changes to the nixos nvidia configuration settings, but there's a lot of conflicting ideas about how to improve them so it hasn't made much progress.
And apologizes for any weird formatting, especially around option names, on my phone.