How to find feedback resistor value?
First, the input impedance is 1.1M because the two input biasing resistors are effectively in parallel (the battery impedance is virtually zero ohms).

Beyond that, you are correct - there are many feedback resistor combinations that will yield the same gain, and there is no perfect value set.  Some thoughts:

The higher the resistance values, the more susceptible the circuit is to noise pickup.

The lower the resistor values, the harder the opamp output stage has to work to drive them; this increases distortion.

The lower the resistor values, the larger C2 must be for the same cutoff frequency.

Back to R1 and R2 - the frequency response profile of an inductive guitar pickup changes depending on the load resistance it sees.  (The pickup is basically a small AC power generator).  The datasheet on your pickup should have information on this.