One neat thing about latter main quest aside from FG would be couple of bloom gifts every stage you clear. And the further you progress, the lesser RNG is the factor and your positioning/speed will be only things that matter. Difficulty seems to be stay the same from stage 15 onward, even at stage 42 the recommend TP is just 480k.
So currently, the end game content would be
1. Whale ship, especially stage with swan boat. (17 onward)
2. Kodaibana, pests are strong and pack more than enough punch to whack your team if they are not geared up properly. They won't give you 10 FG oh so easily after all.
3. Every extra stage of Aqua shadow.
4. The hardest however, would be one from daily quest. let the picture show.
Attachment 6947
800k would be a pinnacle of difficulty for F2P player or light P2P like me.