First idea.
Add an Attack pot
The Attack / Decay of the Envelope should be adjustable by changing the RC Time constant of R22 & C10. By adding an A10k pot in series with the 470 ohm resistor, we change the time constant from 29.4ms to 666ms. If we went all the way up to A50k, we could get a whole 3s rise time and an effect that might sound more like an LFO controlled filter.
The problem that would arise in doing so would be that the R27 would cause loading, and at max 'Attack' (ie the slowest speed) you'd lose half the amplitude of the Envelope signal. We could scale R27 & R21 up to avoid this, but since the following Opamp has a BJT input, that might screw with the bias current. Maybe someone smarter than me could help figure that out.