If you have informed yourself just a little then you know that the money must be in the bank long before extension. And obviously you will have to have a room the day you arrive.
And then: it’s hard to drop money before opening an account.
So there you go:
Rent
Account
Money
Extension
If you’ve prepared well, the first 3 are done on the first day and the last 90’days later. That’s what I did (although I opened the account on day 2 and the money arrived the next day 😉 )
Just make sure that the bank recognizes the money as foreign!!
So you either by an onward ticket right away or wait and see (and maybe need to stand in line twice).
At onwardtickets dot com you can buy one for 10-15 online within seconds.
Edit:
Concerning your retirement.
You need to get a bank account asap and put your 800k in (hopefully they will show as foreign transfer!)
I have Bangkok bank and kasikorn and I recommend kasikorn as 711 doesn’t allow scanning QR codes from others and Apple doesn’t accept Bangkok bank card as payment method in an Apple ID
Maximum queue I’ve ever seen was 6 or 8 people. And the guys help with everything including getting a bank statement that I forgot. He did it in my banking app and sent it to their printer.