Frequency Shifter - A Look at the Input

The goal of this project is to take a guitar input and output a frequency that is half that of the input; dropping the signal down an octave. I was hoping to use a fairly simple technique using an ATTiny1614. After looking at the signal, I think this might be very hard if not impossible given the planned circuit. But I want to give it a go anyway. There is an anti-aliasing filter on the input. It might be the reason the higher notes have such a low output and the filter might be shifting the phase of the overtones.