If all you want is a "soft start" then you can try just adding an "RC filter" to your throttle signal, between the throttle and the controller.
What you want to do is choose a resistor (in series with the signal) and a capacitor (in parallel with the signal, at the controller side of the resistor) that take enough time to charge up the capacitor so it will respond to throttle changes slower than it used to. Use a large value potentiometer instead of resistor (connect only the middle and one leg, leave the other leg open), and you can then adjust it a lot to fine tune it.
The circuit can be made more complex so that it only affects *quick* throttle changes, softening them, while leaving gradual changes alone, or whatever behavior you wish...but the RC filter will probably do what you're after.
What you want to do is choose a resistor (in series with the signal) and a capacitor (in parallel with the signal, at the controller side of the resistor) that take enough time to charge up the capacitor so it will respond to throttle changes slower than it used to. Use a large value potentiometer instead of resistor (connect only the middle and one leg, leave the other leg open), and you can then adjust it a lot to fine tune it.
The circuit can be made more complex so that it only affects *quick* throttle changes, softening them, while leaving gradual changes alone, or whatever behavior you wish...but the RC filter will probably do what you're after.