RC filter on a clamping circuit
You don't need R2 to form a low pass filter from the 12-20V input - R1 and R21 form an equivalent series R to the capacitor C3. Filter cut-off (ignoring the effects of the diodes) is: -

\$f_C = \dfrac{1}{2\pi R C}\$ where R is the parallel value of R2 and R21 (6.667kohms in your circuit).

So, get rid of R2 and simplify things a bit then, tune the filter by selecting the capacitor value to give the cut-off frequency you desire.