Calculate the upper cutoff frequency of a common gate
Your equivalent model is wrong. Cgd should go to ground and so should Cgs.

The Miller effect is only apparent in common source operation where the input is at the gate and the output is at the drain.  The effective input capacitance will have Cgd multiplied by the voltage gain of the circuit (+1).

Cgs is directly in parallel with the input signal so does not affect the frequency response.

In this circuit the upper cutoff frequency is just determined by Rd and Cgd (goes to ground) as a simple first order RC.  Ro is infinite so it can be ignored.

The output of he JFET can be considered a constant current source.  Rd and Cgd are in parallel.

This can also be re-arranged to be a voltage source feeding a capacitor through a resistor. (A current source across a resistor is equivalent to a voltage source feeding through a resistor).