It is now recommended to run the fan continuously on low power, the Antec tricool fans for example have a switch to toggle the speed.
When you are cooling, the back of the fridge will drop in temperature. After the cooling, you also want to move the air around to distribute that to other parts of the fridge.
If you want to control the fan with an SSR, you would do that on the AC side. Your schematic looks correct for that. An alternative would be to buy a PWM fan (4 pins) with a transistor inside that can be switched with 5V directly.