I'm not the best person to ask about the extension as I haven't done one for 4 or 5 years but even so if you Google it it doesn't seem to have changed much from what I have written in my notebook. I did all my extensions at Khon Kaen immigration so it may be slightly different at other offices.

TM7 form

1 passport photo

Copies of: passport photo page

visa

entry stamp

They also used to ask for a copy of the TM6 departure card but I don't think you get issued with them on the plane now so ignore that.

I also had to print my mobile number near my signature on the TM7 form and sign each photocopy and the back of the photo.

I used to apply for the extension about a week before my visa expired. You can probably do it earlier than that if you check with the office you'll be using.

I believe your girlfriend is supposed to supply a TM30 as you will be staying with her. Again I'm not really up to date with this as I haven't had the extension in recent years and so my girlfriend doesn't bother filing a TM30 as I've no reason to attend the immigration office during my stay.

When applying for a 60 day tourist evisa from the London Embassy I write my girlfriends name, address and telephone number in English on a piece of paper, photograph it and upload that as I stay with her and not a hotel. I've done that 4 times now and the visa has always been approved. She hasn't had to provide me with a letter of invitation. I received the latest approval less than 2 weeks ago.

If you submit an airline ticket with a return after 90 days my guess is the London Embassy will approve a 60 day tourist visa as they will be aware you can get a 30 day extension in Thailand and you are showing a return flight. I would try and get in touch with the embassy and see what they say. Don't rely on my guess! I have been in the same situation and the visa was approved but that was about 4 years ago and things change. If the 60 day visa was approved and your return flight was after 90 days then before being allowed to board you may have to show your airline a ticket out of Thailand after 60 days. It can be a cheap one to a nearby country just to satisfy that requirement or you can try one of the rent a ticket websites that are frequently mentioned in this forum. It's possible you won't be asked for either of the above but best to be prepared. The other option is as you say. Buy a flight ticket with a 60 day return and once you're in Thailand change the return date. Could be quite expensive though.