Load at VCC is varying between 5mA to 15mA.
I'd like to solve the circuit to see if either BJTs are in active region.
If that is all you want to know, don't make life hard. 5ma*180 is 900, so the PNP is always active barely.
Increase R33 10x so it's definitely active, then the base current through the NPN is a bee's dick and R36/R37 don't matter much. Though R37 could be a wee bit smaller for lower noise. Currently they will drop around half a volt, which seems a lot.
Your zener is wrong way round BTW. R39 is just a bias resistor for the zener, the low pass filter sees the differential resistance of the zener for AC and a voltage drop from the input for DC approximately. Save Thevenin for when it matters.