I will need to catch the early flight @ 6AM, what time do you suggest me to return the car considering the shuttle bus does not run very fast in the early morning. And do you have one gas station in Hertz as I need to return the car with full tank

If you're a Hertz gold member, you can pull up, hand over the keys, sign paperwork where you've stopped the car, and go straight to the shuttle. It takes less than 5 minutes to complete that process.

Hertz has a return to terminal service where they will drive you back to your terminal for a fee. If you're running late this is a great time saver. They drive you to your terminal in the car you rented or a larger one if needed.

Get gas on the way in and return your vehicle no later than 4:30am

Return your car by 4am. That will give you plenty of time to catch the shuttle, check a bag if you need to, get through security and get to your gate to board your plane. There is not a gas station in Hertz but there are plenty of gas stations near the car rental return area. You shouldn't have any trouble finding one to fill the car up before you return it.