Can I apply for a 60-day visitor visa to Thailand and extend it for another 30 days without hotel accommodation?

Did mine earlier last month with a 90 day return ticket to the uk 2nd time I have done this with no problems

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.

You don't need an onward ticket with a 60 day visa, although your airline may query it. You only need one if arriving visa exempt.

For a visa from the US my girlfriend had to write a letter of invitation....it was in English. She signed and scanned a copy with her Thai ID with it. Never had an issue on that.

That's up to your airline . I've never been questioned coming for 60 days but arriving on visa exemption 30 days ( Aust. Passport) . I've never had my partner write anything . They did call her once ( during the pandemic as I put down my Quarantine hotel details on my TM6 card when they had them )