Need help deriving cutoff frequency of output (collector) RC network
User1726, your start is not correct.
The cut-off frequency of the first order highpass is defined as the frequency where the magnitude of Vout is 3dB less (factor 0.7071) than the MAXIMUM of Vout (which is Vout,max=Vcoll*RL/(RL+Rc).

There is a simpler method for finding the cut-off frequency. You have nothing to do than to find the time constant of the circuit (simple visual inspection) which is T=C3*(Rc+RL). The inverse of T is the (angular) frequency for cut-off.

And don´t overlook that the capacitive impedance is 1/2*Pi*f*C (and not 1/2*Pi*f)