Gonna straight up say here, middle difficulty is not going to work like people think it will.
I played another game that decided to split major content into the three difficulties you describe, and what happened? Everyone immediately disregarded the fact that the middle difficulty even existed, to the point where the devs reversed that decision not even a year later. No one would even admit to only being able to clear the middle difficulty instead of the hardcore difficulty, even though the hardcore difficulty was pretty much deliberately designed to be even more total bull[censored] tier than Gordias ever was because Korean F2P MMO.
Said game had a much smaller raiding community than FFXIV. One must understand that the devs method of assigning value to content is far different from what value the community itself will assign to it, and it is quite frankly something they can't control. Hell, I'd go as far as to say that the vast majority of people who complained that the end of Coil was too easy were just grandstanding on message boards, living vicariously through the progress of other raid groups, and didn't ACTUALLY clear the entire coil themselves. One only needs to see how people reacted to world first groups clearing Creator so quickly to notice that - a 48 hour clear later and people who weren't even related to those groups already declared the content too easy.