Can I enter Thailand on visa-free entry and convert to a Non-O retirement visa later?

Requirements for visa exempt arrivals is, Onward travel and Funds of equivalent of bath. Get a cheap refundable ticket and after entry cancel it. BUT as previously mentioned. To be sure and peace if mind, apply from home.

I arrive on Visa free entry, go to immigration without an agent and change to type O visa based on 800K Baht in Bangkok Bank. I had BBL London account for 2 years and later at BBL Bangkok. They gave me 3 months extension of stay and then another 30 days to make sure I was not illegally using an Agent to fund the 800K. Then I got the first 1 year. Also did the same after I was working overseas and could not make visits to immigration to extend the visa. Been here 32 years off and on without problem, except for the visa agents doing illegal things to get people who should not qualify to stay here long term.

Buy a cheap onward ticket within 60 days, then you'll be good for boarding your flight and Thai Immigration.

I fly about 10 to 20 times a year into Thailand i would estimate i get questioned about return flight 75 % of times now (i have visa so show that) prior to covid probably about 5 %. Your always going to have people say i did it but the likelhood now is very small. Thai goverment has made airlines responsible if you cannot enter the country.

Yes you can get an agent to sort your non O + the 12month extension, but you will save some money if you get the Non O before you arrive sort your bank account out this is very easy to do yourself then get the agent to sort your 12 month extension. You will save at least - bht

How much?

You can always purchase a refundable ticket...or an onward ticket on a bus

The 'rules' say you must have a return flight within the duration of your stay and as you can't get an extension up to 6 months on a 'visa free' entry then I really don't think you would get in and might not even be allowed to fly as the airline would probably be responsible for bringing you back if refused entry. Really you need to sort out the non-O visa before travelling and then 'convert' to a retirement extension when here. When I came over 14 years ago the woman at the airline check-in desk didn't notice the non-O visa and wasn't going to let me fly as I didn't have a return flight booked, I was OK once I showed her the non-O visa so that's an example of what might happen to you.

Yes you can, and why need an agent, easy to do, a lot of immigration officer have volunteers or people to help you

You can't get a real visa on a visa exempt

Yes quite likely

You could just get the visa before arriving.

Ask your airline they are the ones most likely to have a problem with it. You might be better just getting a Multiple Entry Tourist Visa (METV) if you want to stay 6 months, with a border bounce, but will need to show a flight in, travel out and travel back to Thailand to justify it