Enemy mobs don't get stronger in other dimensions no matter how far I travelled (tried it in Nether and End).

I got BetterNether & BetterEnd installed, if that matters.

do you have default settings enabled?

Also faced with such a problem. Created a datapack and specified the address according to your guide (data\rpgdifficiency\difficulty\YOURFILE.json). But for some reason mobs in other dimensions do not change.
Maybe I need some additional settings?

I also use BetterNether, BetterEnd and The Twilight Forest.

it would be awesome if you could just have a slider multiplier for each dimension so for example i could have a 10x multiplier for the nether and then when you enter the nether its a the same difficulity as in the overworld

I accidentally used a different id for dimensions than the world registry key. Will be fixed with version 1.3.6