Adjusting Voltage regulator with PWMed optocoupler
You'd better to start again from scratch. Your servo for sure doesn't need a linear PSU, you could use a buck converter for that purpose. Next thing that you do want to have is a LDO with PWM adjusting as it makes a PSU with lots of noise and lots of heat dissipation, a useless power supply. Further it is not clear why the galvanic isolation is needed.