Switched Capacitor with MOSFET
The MOSFET you have chosen (1RF530) is not at all suitable for a high frequency circuit.

It has an output capacitance of several hundred picofarad even when it is off. It is not going to make much difference changing the bias.

You need to find a device whose output capacitance is low relative to 1pF - I don't think you will find one. You may be able to use a diode switch to do what you want.

There are devices available that have electrically switchable capacitors or maybe a reed relay.

1RF530 datasheet

Digital Programmable Capacitor