I havnt reached Lv70 yet, so I can only mention prior stuff.

but imo, the "propper Difficulty" should be akin to Sophia EX/A9S/Ifrit HM/Garuda HM/Titan HM. (And I mean for when they were released, as Ifrit HM cant 1shot the entire party with eruption anymore.)

There should be something semi challenging for the people who arent quite as good, to try and achieve.
Then right after, the 2nd EX primal should be harder, but essentially the pinnacle of what that player could ever achieve, if they played the best they can.

for Savage,
A9S is a good introduction, that feels like an EX, and also decent for new players who want to attempt savage, but wont ever get past the 2nd floor.
A10S, is semi harder than A9S, and a good design.
A11S is where you start to see content that requires the best players, even if its still on the easier side for them.
A12S... is terrible.
Its roughly equal in skill requirement to A11S, to some more and less.
It should be more akin to the 3.0 difficulty of A4S.
The final battle should always be the hardest. Something even the best struggle with.

Hopefully for Omega, O1S-O3S is roughly equal to A9S-A11S.
But we also know another harder savage mode is comming out, with one battle.
Hopefully its the 4th floor, and more akin to 3.0 A4S in difficulty.