I had a three month visa obtained in uk but intended to apply for retirement visa when I arrived to stay indefinitely. I was worried about not having a return/onward ticket so I emailed Thai embassy in London who replied quickly to say I did not need proof of onward/ return. Kept a printed copy of the email when I travelled but was never asked about this.
I arrived on 90 day then once I had opened a bank account I transferred from my wife’s Thai bank account. As many have stated it may depend on the individual person at immigration to accept.
For example ( this is a bit off topic) When we arrived in Bkk airport my wife and 12 yr old son have Thai passport. I had uk with 90 day evisa printed and expecting 90 days stamp. The immigration officer said to my wife “ you and your son are ok but for your husband I will need some whiskey “
She told him to get lost and he stamped my for 90 days.
What I’m trying to say is you never know what to expect.
We found immigration at the office out of town in Hua Hin extremely helpful. The one in town in Blueport not at all helpful. Many banks told us not possible to have a bank account but opened at another BKK bank on same street. If you dress smart ( I don’t normally) and be very polite it seems to go a long way with Thai officials
I did mine myself in Hua Hin ( main office, not Blueport ) and they could not have been more helpful. There is a lady at the desk downstairs who gave me a checklist to work with ( about 4 items as I remember) They have a separate office on-site who will take the necessary photos and photocopies for a small fee. If you have a little time and transport it’s not difficult. I suppose if you’re in Bkk or Pattaya with heavy traffic and no transport it could be a pain to do it yourself. Here’s a business card to get a bank account in Hua Hin , very helpful, I declined the “recommended “ insurance and they were fine. It was two trips to the bank but not difficult. Make sure it’s this branch, other Bangkok bank branches told me not possible to open an account for foreigner. Ask other expats where they opened an account
I arrived back in UK from Thailand in July with a German passport expiring in 15 days and nothing was said So with UK passport surely there is no problem
I just tried 10k at a time to check it works as I need to spend a lot of my 800 while here and going back after a year. At least I know that if I still have a few 100k I can send it to uk.