As the title says. Is there any way to increase the number/difficulty of monsters spawning.
I'm currently playing the Craftoria Modpack and im kinda overpowered with my gear and world tier pinnacle and oneshot everything ¯_(ツ)_/¯.

For mob strength, you can use a datapack to create new tier augments or buff the existing ones https://github.com/Shadows-of-Fire/Apotheosis/tree/1.21/src/generated/resources/data/apotheosis/tier_augments

Apoth does not have any mechanisms to increase the frequency of mob spawns though, you'll need to look to other mods for that.