While I am new to this as well, I can tell you what I have learned in order to set up my first system recently.

The depth of your grow bed should be around 12” deep in total.

The level your water should reach is about 2” below the surface of your grow bed, around 10”.

I am not certain on if synthetics can be used, but I don’t see why not as long it’s a stable material.

You should pump the entire content of your fish tank once an hour. Lots of people do ebb and flow systems and get the pump running for 15 minutes per hour.

I am not sure about the science as to how the bacteria are kept in the growbed, but I’d assume they can get into very tight spaces that make it hard to get moved. They are very necessary parts of the system. They convert the toxic ammonia in your fish waste into nitrates that can be consumed by your plants. Without them there is just a constant flow of toxins to the fish and plants.

I use LED bulbs that have UV spectrum. Pretty much as long as it says it’s a grow light, and it’s the right wattage for what I need. I am not the most experienced in this part.

I am not sure about the last question, but look forward to seeing an answer to it!