To your point, the easy-hard concept is so vague. What does that mean? Less enemies? The same amount but they have less hit points? I take less damage? Longer time before enemies react? It can be literally anything.

Let us control the settings like you said, and then have some presets for easy configurations and starting points.

I think a special shout out needs to be made to the sniper elite series, which let you reduce the difficulty of the enemies but keep the difficulty of the sniping mechanics.