there are mods that do things, but your best bet is either starting over or trying it with reduced food, reduced armor, reduced weaponry, etc.
you can also try all mage armor/etc, with low armor and health its harder than mixing 1 eitr food in & fireball here & there.

Or try a grand building project.

Difficulty settings are on the devs' list of things to do, but they're not here yet, and it may be awhile.

Valheim Plus can make things harder by letting you change just about any number in the game. Reduce armor or damage, reduce resources. increase enemy damage, etc. You can also try a no map or no portal run.