First of all, look at how much your GPU is utilized while gaming. If it sits at 98-99% all the time, yes, then it's doing max performance - which means it will get hot! It doesn't matter how the game "looks" - it matters how much it is utilizing your GPU.
Secondly, 76 degrees is a perfectly fine temperature for at fully utilized RTX2080S.
Valheim is indeed GPU heavy - and if you turn on all the bells and whistles it will utilize your GPU to the max.