Maybe there is an actual answer, I don't know, but what I do when I want to take screenshots is zoom in as close as possible, and sit down. That should make the character turn invisible due to camera angle being too close. If you capture the screen at the right moment, you can take a screenshot without the camera moving down to sitting level with the character as well.

works great, looks great:

https://www.nexusmods.com/valheim/mods/44