Given that literally nothing MostHost said came anywhere near addressing your actual question, which was about slope based masking (except, I guess, ‘bake out a slope map in an external program,’ which isn’t super useful advice), can I ask what issues you’ve had with whatever youtuber solutions you’ve tried are?
I knocked together a pretty basic auto slope mask that takes a Degrees input in a few minutes:
It’s slightly more expensive than a SlopeMask, with 209 (208 if I drop the Rad2Deg conversion) instructions vs a SlopeMask’s 207, but if two shader instructions is the difference between a playable game and not, then you’ve got much bigger problems.
There’s also no transition softening with this setup, but again, knocked it together in a few minutes. There are a few possible options to blur it, but they come with different expenses.