Soft start circuit design for audio amplifier
Overall, ok.  But ...

R1 is a bit large.  Decreasing it to 10K will increase the zener current to a little over 1 mA, a better number for stable regulation.

R2 and R3 form a 2:1 divider that decreases the M1 max. gate voltage to only 4 V.  I would adjust R2, R3, and C1 so the max gate voltage is in the 8V to 10 V range, while maintaining the time constant you want for the turn-on slope.


The gain of M1 acts to speed up the voltage slope at its gate, and output transistor M3 speecs it up even more.  The output voltage ramp might be faster than you expect
You can eliminate M1 by placing the timing components around M3.  C1, D1, and the discharge resistor between the gate and source, and the timing resistor between the gate and GND