Fan control using oscillator with varying capacitance
Two hands about a foot apart is less than 1pF and with inverse distance vs C you need a fairly high f and low C LC resonator to modulate the frequency. Theramins used a thin insulation over the electrode surfaces of a Unijunction oscillator to create this sensitivity over many decades of f using the small [mm] modulation of hands near the surface using high voltage and high R, small C.

Back in 1970, my friend used a waving hand with just 60Hz to couple the stray E field thru buried plates to detect the E field into 10 MOhms and could modulate the light intensity using a logic cct to drive a staircase generated phase angle on the triac.

So it depends on your impedance and the stray E field noise nearby. You can use high f or low f but it depends on the interference levels.

But to increase f modulation or to detect the level of induced E field,  C fixed must be as small as the smallest  C variable of 1pF, so coax cable is out and ground capacitance must be minimized between electrode plates but each plate can be insulated with thin plastic coating.

It is best to use 10k series R with ESD protected CMOS for high impedance input stages to detect E fields. Thus use 74HC14 or similar not TTL 7414. And the series R is to limit ESD current.

It is possible shape the attenuation of unwanted noise with filters. But to make a relaxation oscillator sensitive to 1 to 10pF changes in frequency, one needs a very high R feedback into the Schmitt inverter oscillator. such as 50M * 10pf = T = 50us then scale this frequency down using a CD4060 binary counter to use the frequency of the output to control the on time of a fixed PWM driver such that the highest frequency resets the PWM one shot the quickest and the lowest frequency gives the widest pulse using reset. More clever ways can eliminate aliasing effects.