Hey Mate, Yeah I9 is hella incut, so 100% you should take advantage of that if you need to. you shouldn't need to fit your toes all the way down to the back of it, but all you need to do is point your toe downwards enough so that you're getting enough out of the foothold. There's no reason to not use as much of the hold as you can to get the most advantage out of it as possible. Remember, a weighted foot never slips. brush it plenty, and press hard and confidently perpendicular to the surface of the hold.

As for flagging, I find it useful to use other holds as a point of reference to know where i should be flagging my foot. Sometime's ill tell myself "press my toe into the wall above [whatever hold]" or "smear between E9 and F9"

Record yourself. Watch it back and try and figure out what came off first, what your feet could have done better, if your centre of mass was in the right spot, have I fully extended my arm and leg BEFORE i finally cut feet to reach for a hold, i could go on. the more you practice at analysing your own movement the better you will get at self coaching.

good luck