This doesn't add an actual difficulty you select when starting a character, does it? Like between High Fantasy, Dark Fantasy, Hardcore. I've been wondering if it's possible to modify those or add to that list, but it sounds like this just applies over top.

Yes, they are compatible. The x3 or x6 mods updates the locations.json file, whereas the difficulty mods directly modify the attributes of each individual monster's json file.

Tried it. Everything works great with mods for enemy spawn rates.
Overall, a good mod that increases NPC strength...))

Are this mod and the spawn x6 mod compatible?
I understand that you developed both, I'm just curious)))
These are the first two mods that really interested me)

This mod will make enemies VERY tough. Health, armor, speed and attributes are all increased.

This is a global change for all entities that have an AI component. So even villagers and animals will be more difficult, if you choose to attack them :)

Health: 1.3
Armor: 1.3
Attributes: 1.3
Magic Power: 1.3
Ammo: 5x, so no, those archers are not running out of ammo
Speed: 1.10 (escaping is a challenge)