First of all, congrats on your ACAS! As someone mentioned, your personal career goals should be considered so advice really needs to be customized for each person. For me personally, I’m still trying to get to FCAS, so order of exams never mattered other than which ones were offered when. My first fellowship exam sitting was Fall 2018, so my only choice was to skip or take Exam 8. I took Exam 8 but didn’t pass. So, you have to consider also when they will be offered (again as the non-Robot mentioned above 🙂 ) 7 and 9 are not on the Fall 2021 Exam Schedule. Without knowing anything else about your situation, I’d also guess Exam 7 for Spring 2021 is your best bet.
Regarding which ones are harder: For me, I found Exam 7 material harder to digest than Exam 8, but it’s different for everybody depending upon how their brains are wired, their experience, etc. Take that with a grain of salt, though, because I haven’t passed any fellowship exams yet, although it took me a LOT more time to get through 7 material on the first pass vs 8 material. Also, I haven’t really studied Exam 9 material yet, so can’t comment on that. The weird thing I’ve found over the years with these exams is that difficulty ranking of the material itself doesn’t necessarily line up with difficulty ranking of passing a particular exam.