No man fears what he has seen grow
I usually just prop my hand with my other hand. Had to do that when I drew a lot.They have mounts that help keep your hand still. There's also a few techniques involving the contrast paint that will also help.
I used to do all my own paint jobs on my own miniatures, I just haven't painted any for a decade. I'm a perfectionist so I hate doing a sloppy job on anything so I take my time. Which also isn't good if you have any nerve damage. My hand starts to get sore after an hour, my arm after two.Certain models like skeletons or necrons require very little fine motor control to paint to a reasonable standard.
For a skeleton, if you simply paint the whole model a bone color, and apply a brown wash, followed by an eggshell drybrush and a single layer of silver/bronze for the weapons, with some dabs of orange/teal for rust or verdigris and fonally some brown for the weapon hafts, you're good to go, and you can be as sloppy as you like.
Nids aren't much different. You really only have to be able to paint the flesh and carapace separate colors.