Question: why is the zero cross detect needed as a separate piece of circuitry? Aren't most all solid state relays equipped with zero cross optocouplers internally?
Mark S.

The zero cross detector is used for doing phase angle control modes with a random fire (instantaneous fire) SSR.

The Arduino needs to know when the zero cross event occurs so it can then switch power on at a certain number of milliseconds later depending on the required power output power.

I am using an AC fan, leaf blower motor. I was planning on manually controlling it esp as the ZCD is currently out of stock. Is this something that can be added later?

I have a display for arduino... don't know if it can work or not. I need to do some learning on it.

As long as you have speed control of your motor. A ZCD and SSR can be added later. It may just require changing the mode used in the TC4 sketch depending on what you already have on it.

I should be building more ZCD's on the weekend hopefully.

You can potential use the LCD as is by connecting all the required pins to the arduino but I have not actually confirmed if that still works with the TC4 sketches. I'd recommend getting an LCD with an I2C interface. It will make the connection much simpler.