Work on one thing at a time... android phones is another language, but what you would likely have seen is an emulated com port through Bluetooth, after all Bluetooth is just wireless serial that can drop packets,
As for one or multiple arduinos, it comes down to how well you write it, No matter how you approach it, you will likely need a form of PID to keep the thing stable, now one thing to know is that the normal uno does not support floating point natively, so its slow to run, there are ways to run approximations much faster, but there is some heavier math behind it,
As for your inputs and outputs, like a previous poster said, get familar with the timers and counter peripherals on the micro, and let them do the heavy lifting,
My own attempt started on a due, simply because it was much faster,
In both cases (uno and due), the IMU and PID didnt fill or overload the single micro, but like many before me, i started to stall out from lack of not seeing immediate results infront of me,
I've attached what i had, not sure how much of it is useable as i abandoned it some time ago, but see if any is helpful, pretty sure i went a bit overkill on the self cal,