If youre determined to go through with this in the face of everyone telling you it isnt necessary, then shoot for Ultima then Ifrit. Ultima is arguably the easiest, it doesnt have a ton of tricky mechanics, and it isnt super uncommon for a PUG to clear it if you get a DF que to pop. Ifrit is a good medium difficulty. It takes the new mechanics from hard mode(along with the original story mode ones), adds one or two twists, and hits a little harder. Avoid titan(frustrating), leviathan(frustrating), and ramuh(probly the hardest one).

With that said, you might be operating under a false assumption. You seem to think that this game has a "standard" difficulty curve for its story missions. It does not. The difficulty "curve" is more like a flat line with an odd instance here and there that suddenly spike upward, then settle back near the original level. True the level 75 story dungeon might have more complex AoEs to dodge then the level 20 dungeon does, and classes get more complex as they level up, but the general gameplay isnt significantly harder. Dodge telegraphs, heal through roomwide attacks, and attack the enemies down. If anything, the added class complexity makes many of the fights easier(shifting the difficulty from what the dungeon does to doing your own thing properly). Tanks have more defensive moves, healers have more "emergency" buttons, and DPS have more and better attacks to hit with.