I've beaten it, I posted the video in another thread. Here's some tips.
You can't crank the music box unless both Penny and Carmen are not in your vents.
Bjorna let's you crank the music box when her jumpscare is primed. The strat I did was 2 stages of crank and then go for a flash repair. The repair will happen in the background while she's trying to get you.
The bongs you hear indicate either Bjorna, Carmen, or Annie at the front entrance. You want to check these almost always and as soon as possible(though I didn't always because I believed it was Carmen, since she just reset) Multiple gongs can happen at once.
If Annie is in the hall, always attempt to do her minigame, even if both the animatronics are in the vents (if you let Carmen or jumpscare Penny you while Annie is there, you lose ) 99% of the time you will not get attacked by the other animatronics once you start flashing Annie.
Once you do the Annie minigame, you have a small window to crank the music box (one to 2 stage cranks) b4 the other Animatronics try to jumpscare you.
2 stages of Mary is also about the time of the vent sound, which you can time to basically get an instant jumpscare attempt instead of having to wait and getting good repairs in.
I hope these help whoever is trying it. Not a whole lot of rng for this strat, but you might lose a few times