just a quick warning Peter. I see this question is getting many replies. Don't be confused. You need a document about where you go after quarantine hotel. I would stick with your original plans. Thai immigration are not stupid. If you don't have a onward flight they are less likely to require a return ticket. Also they are aware of people arriving hoping to get a covid extension. It's by no means guaranteed. I'm in quarantine right now. Good luck!
In this section for your situation you put proof of your hotel booking after your quarantine hotel. So where will you be staying after quarantine? And what proof do you have?
Arrived on Thursday you will be ok. But a message to anyone going now it is very serious in Bangkok now a ND believe me all your paperwork is checked thoroughly. Mine was checked 3 times. Thry are particularly interested in your coe insurance documents and negative virus tests