You don’t need to show a ticket out of Thailand or insurance and you don’t need paid accommodation for the extension
Most offices will let you extend upto 30 days before your stamp expires and it’s added on to the end of your current entry stamp regardless of which day you go
not now the test and go has finished, all you need is a flight number and give an estimate of how long you will stay and an address where you will spend your first night, you don’t need anything booked or confirmed apart from the insurance.
I use an annual uk travel insurance you don’t need Thai insurance but they do recommend one for ฿650 for 30 days.
TP is very simple to get if you have a vax record and travel insurance, they are approved in a matter of hours, you can do this before buying the plane ticket.