There are other people struggling with sddm here: Sddm fails to launch
Can you share your logs?
You’re also notably missing the modesetting option which I think is required for wayland.
My suspicion is indeed that you’re not disabling the iGPU properly, laptops almost certainly have more than one GPU. You’re both forcibly configuring nvidia with forced kernel modules and not disabling intel/amd, so this will probably cause issues.
If you want to disable your non-nvidia GPU, take a look at this nixos-hardware module: nixos-hardware/common/gpu/intel/disable.nix at 95c3dfe6ef2e96ddc1ccdd7194e3cda02ca9a8ef · NixOS/nixos-hardware · GitHub
Check out the others while you’re at it, there may be one for your laptop, and maybe you actually do want to use both your GPUs and not burn through battery like it’s nothing. The nvidia modules are helpful anyway, since they set up a good basic config that should work™.
Also, while I’m at it, you almost certainly want to enable the powermanagement service, without it sleep won’t work. Finegrained is useful on laptops, too, without it any dGPU/iGPU work splitting is pointless. The env variables are wlroots-specific, so pointless for plasma. In general a lot of your config is redundant at best, you should definitely simplify it a lot to get it working before tweaking details like this.