You forgot to mention sticking a skeleton spawner in a cardboard box, taking it to the nether, and setting up a wither skeleton spawner (which is what I did to fill my soul shard.) Much quicker than relying on random wither skeletons spawning.
I saw the box used in a video recently but didn't know what it was till now. I usually use a dolly to do this. Need to look in to this cardboard box thingy. Thanks.