I would recommend to remove most code from the interrupt handler. Printing to serial and doing analogRead or are not a good idea in an ISR. Set a flag, leave the ISR and then control the rest from your main loop.
I also recommend you rewrite the main loop. Do not use while (central.connected()). Your loop should always run as fast/often as possible. That way you can add more functionality to your main loop without breaking it. Right now your main loop is no loop when you have a central connected to your peripheral.
Maybe the following example I wrote some time ago will give you some ideas. Reply #2
https://forum.arduino.cc/t/bluetooth-communications/902610/2