You could mod, but you can just as easily do a number of things to make the game harder if you want to. Use weaker weapons is the easiest. Doing less damage is essentially the same as increasing the HP of enemies. Don't upgrade your weapons, same reason.

Use lower quality armour and don't upgrade it. Eat weaker foods, or eat only two foods at a time instead of three. Doing both of those will lower your health, defense, and stamina making the game harder because you won't be able to take as many hits.

Run around berserker style, shield, weapon, pants, no chest armour, no helmet. Go to a harder biome. Don't use mead to heal. Run around without the rested buff. Use only melee or use only ranged attacks. Use a spear but only attack by throwing it. Carry only one so you have to recover it before attacking again.

Loads of options.