Not sure about adding mobs/loot, but Valheim Plus is my must-have mod. It's like 30 QOL mods all mixed into one, but the biggest thing about it is that you activate each thing individually. This means you can tweak tons of things exactly how you like it without it being as rigid as other mods.

For example, want to make mining more bearable? Why bother searching for other mining mods when you could specifically increase durability of only pickaxes, reduce stamina cost to use pickaxes, and specifically increase the drop chance of mined resources? You can do that with V+.

Want more difficulty? You can tweak things like having a multiplayer multiplier on at all times, decrease exp gain, make food degrade faster, make items weigh more, have fewer inventory slots, decrease durability of weapons/armor, decrease defense values of armor, and much more. Whatever "increase difficulty" means to you. All options in V+.

And all of this is changed in a simple text file in notepad, with each section clearly explaining how to do so, and how your changes affect the game. Very user-friendly.