My own experience is from two arrivals with 14 day ASQ hotel stays then at my condo. There’s a pre-immigration document check and they wanted to know both the hotel and the onward address. One traveler kept repeating he didn’t know where he was going to be staying after ASQ and eventually the officer ran out of patience and sent him to the back of the queue and told him to make his mind up where he was going to stay.
when you arrive in Thailand, immigration will ask you for your onward travel stay intentions, so have the address details on hand when you arrive too.
Just as an aside: I have arrived twice under Covid restrictions. The first time in January and a “free spirit” gentlemen told the documents pre-check officer he didn’t know where he would be staying as he was going to “travel around”. The officer very politely repeatedly informed him they required him to give an address. Eventually the officer told him to go to the back of the queue and come back with an address.
I have never heard anything about 21 days in a country and I wasn’t asked that. Why would you need a single tickets? You should check your home country Thailand Embassy website.
without wishing to inflame the anti CoE legion, I have entered Thailand twice in the past ten months and completed the CoE both times, and I honestly don’t know what all the manufactured outrage and indignation is about? It’s a precheck before travelling and ensures your paperwork is in order before attempting to travel.