Plus, I'm pretty sure it's fine to take your time to beat all levels on 9.0. It is a game. As for me, I'm going through and beating all the levels on 7.5+, when I have time. It racks up the hearts and weapons, and I can always go back and do more 9.0 runs when I feel like it.
For me it was more like when I get around to it
I did most chapters on 9.0 on either the first attempt, or the second/third for one reason or another.
The one that gave me the most trouble was Chapter 19, since you need to do that one with a Club. Easily the chapter I was finished on the most times, more than 5 I'm certain.
Second most problematic for me was Chapter 11 on 9.0 with a Phoenix Arm. God I hate arms, Clubs are tricky to use and the Earthmaul club isn't exactly exceptional, but they have their potential in better skilled hands. That chapter took me three tries.
Chapters 18 and 23 I was finished once, Chapter 18 was because Pit's Body is very dangerous, Chapter 23 because I got critical and wasn't able to take out a Firewyrm towards the end, that thing is actually very dangerous.
Chapter 24 I somehow cleared first attempt, go me. Same goes for a number of chapters on 9.0 actually, like 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 13, 14, 15, 16, 17, 25.
Chapter 7 I tried to clear on 9.0 early, wasn't able to for a while but did eventually. Chapter 20, I'm not sure I cleared that one on my first attempt, the last enemy wave is tough without a strategy to go on. Chapter 21's air battle is the hardest part, far harder than anything the land battle throws at you. Chapter 23 is also just balls hard, especially with Hades Heart.
Let's see though, my times finished count after all 9.0 clear was just 44. What a refreshingly challenging game!