Increasing ESD protection on Isolator part of the board
To improve ESD protection you need a way to stop the discharge getting to the sensitive silicon. There are multiple ways to do this, you can either stop the signal, or put the signal back where it came from:


Series resistor: this is the cheapest way to do the job. It adds resistance and parasitic capacitance and inductance. This reduces the spike coming along the line. However this will have limited effect as it is just a resistor, and so will also effect the signal you want, as it is a resistor.
In line inductor: which will block (greatly reduce) the path for higher frequency signals, which is what an electrostatic discharge is. However this will block high speed signals which you may want, and is more expensive and takes up more board space than other options.
Capacitor to earth/ground/return line: this gives a route for high frequency signals to go back where they came from. Selecting the capacitance value and voltage range is tricky. Also, the package matters, 0402 capacitors will work a lot higher frequency (due to lack of parasitic inductance) compared to 0805 for instance. This is probably cheaper than inductor, more expensive than resistor.
Protection diode: this are silicon devices which are designed to do what the capacitor is doing, but only for high voltage signals. These are the "proper" way to do it, as they are designed for the task. This means they have the least effect on the signal, but also they cost the most. You can get TVS diodes, which are often designed for voltage spikes rather than ESD, so lower voltage but higher power, which also react slowly. If you get a very good ESD protection diode, it will cope with the high voltage and react quickly. When looking at these, don't forget to consider the diode capacitance will may effect your signal in ways you don't want.


I have used all of these for different reasons at different times. My go-to option is the protection diode. However, ESD is a very quick impulse so usually I do a protection diode in parallel with a suitable capacitor.