Different ways to implement soft start in SPICE modeling?
Essentially a soft-start circuit limits the output current during some time at startup. It is usually accomplished by charging a capacitor with a given constant current source (like you are doing already).
If you want an even simpler solution, you could try the following (LTSpice):

How it works

B1 modulates the soft start's constant current source + capacitor. At startup the current is limited to the required limited output current. Once the time out expires (SS capacitor fully charged), the maximum current can be delivered. The resistor R3 modulates its resistance in order to have the allowed current delivered to the load. 

In this circuit you just have to change the following parameters:


SS_timeout
Soft Start Current
Maximum output current