So many people blame Valheim for heating up their PC. It's not Valheim. It's your hardware and airflow trying to tell you something. No amount of optimization is going to reduce temps. They are what they are because of your setup.

You could go crazy person like I have and mount some more fans into their PC in an attempt to cool the more critical parts. I did that and it works good. All my temps rarely go above 30c because I one night I got sick of high temps and just mounted fans everywhere temps were high. It's not quick or easy solution but and you end up with a Frankenstein PC, but on the plus side you get the future proof cooling you need.

Bottom line is case fans are much cheaper than burned out cpus and motherboards and heat issues in general. Order a bunch of 5 dollar case fans and plug them into your power supply. And the more cooling the better. Cooling extends the lifespan of everything in your pc. If you can live with the extra noise, well it's the sound of sustainability.

Lastly, get a few cans of spray air dusters and spray out ur pc. Don't get too agressive tho cuz I've actually wrecked my pc doing that.