Some people just chance it, if you get asked for a return ticket, just step away from the check in desk, and book the cheapest ticket to a neighbouring country, many times you won't even be asked, so i think buying a ticket in advance is a waste of money.
you still need a Thai bank account, for a 12 month extension based on retirement, even if the agent helps with the financial side of things, because the bank statements need to be in your name.
What makes you think there is a DTV card?? You just print the pdf visa, that the embassy sent you in an email, and present that with your passport when entering Thailand.
Get a retirement visa for Thailand for yourself, and you can spend extended periods of time in Thailand, without any problems coming and going, try living together first for extended periods of time, take it slowly, don't make any big purchases, you will likely get requests to buy a house, car, motorcycle, gold, give money for her family etc etc, wait until you are more sure the relationship is genuine, some will play the long game, you can get a 6 month visit visa, for your girlfriend for the UK.
there are agents in Pattaya, that do the initial non O visa and the 12 month retirement visa extension, for between ฿25k - ฿30k and the renewal every year is between ฿13k - ฿15k it's extra if you need a bank account opened.