I got the point of 1000 instances and 300 commendations.

But you can achieve that quiet easy ignoring some old content. Remember you do this sync to the original lvl. I gonna do a short list of some that could be hard if you have never done it on the original lvl before.

Maybe with the trial roulette:
The Chrysalis (the lb part)
The Steps of Faith

Only via mentor:
The Howling Eye (Extreme)
The Navel (Extreme) (We know the pain)
The Bowl of Embers (Extreme)
Thornmarch (Extreme) (No way to help)
The Whorleater (Extreme)
The Striking Tree (Extreme)
Akh Afah Amphitheatre (Extreme)
Heavensward all primals to in EX when stormblood come out.

Btw I have done some extreme on my mentor roullet so is not impossible.