555 timer Theremin - capacitor choice, and question about mixing signals
I don’t have Theremin recommendations at this time but two fingers about a ~ cm apart is 1pF then inverse squared , so change the wire to a small area coil or foil.  You can mix 2 100kHz oscillators in a XOR gate or diode mixer.  Try that.
Capacitance depends area and gap. Then 50pF fixed is about right if the same f.  But stray capacitance between sensors may cause both to “lock”. Injection locked loop (ILL)
Report back with photos and sounds when done.