Ok, I’ve got that working on my GT9400. A little confusingly there are two sets of instructions on this forum. The ‘official one’ is [HowTo] get legacy 340xx NVIDIA drivers back where you use the pkgbuild from nvidia-340xx-dkms/PKGBUILD at master · philmmanjaro/nvidia-340xx-dkms · GitHub.
As you can see from the pkgbuild it only has patches
but should be ok since 5.10LTS is supported until Dec26. If you use this ‘official method’ you’ll have to stay with those kernels.
At an upgrade to nvidia-340-dkms replace the pkgbuild with this one nvidia-340xx-dkms/PKGBUILD at master · philmmanjaro/nvidia-340xx-dkms · GitHub and edit line 9 replacing pkgrel=1 with the version you want to upgrade to (at the moment 32, I believe, so change to pkgrel=32) and save.
I know there is a newer set of instructions on this forum where ppl had success for all kernels but I’ve never tried that, just search ‘340.108 5.15LTS’.