I have done this journey a few times. If both flights were booked at the same time. You will do immigration at Bangkok, but not the main one. You follow the signs for Phuket, and that will take you to immigration, and into the domestic terminal. The main clue will be if your luggage is booked through to Phuket.
They will probably make you use the visa on your first entry. Don’t think you can get 30 day extension, but there is a possible extension for visiting family.
If you do the 800,000 for the first extension, you can deposit 65,000 a month during the first year, and the use that for the second extension. You can then withdraw your 800,000, and reinvest in to earn interest. You can get interest on your 800,000 during the first year, but only a very low rate.
The bigger impact at the moment is the exchange rate, and what that will be at the time you wish to come.
You are on the wrong site, there is no payment. Find the official Thai government site. It is free to do, but not more than 72 hours before you arrive.
I have done 7 visa exempt in a year before, but only stayed 10 days each time. Never had a problem. But I would recommend getting a none O before you come. Makes everything easier.
I have just applied today. I put down the address of a hotel I have stayed at before. I have had a visa before, they do not check the flight, or the address.