Trouble with optocoupler 4n35
Driving a MOSFET directly from an optocoupler seems a poor idea, especially for PWM operation, because you won't get the switching speed you need.

Dependings on the MOSFET, the voltage and current, the grid intensity needed during switching can range from hundreds of mA to several amperes. This is required to shorten the switching time and thus limit commutation losses.

With your design, during tur-off, the current from the grid is limited to:

$$ \frac{20V}{10k} = 2mA $$

This is orders of magnitude too low.

Also consider that, even during turn-on, optocouplers tends to be slowed down if the load resistance (10k here) is too high. I'm afraid the value of the resistor does not fit your needs.

I definitely think that using a MOSFET driver is a good idea. If IR2110 - like drivers do not work for your project, maybe you could consider components like MCP1406/07 ? They can be seen as a powerfull digital buffer/inverter, that can drive a MOSFET.