What advice do you have for obtaining a visa to stay in Thailand for 6 months without risking upfront costs for flights and accommodation?

Not sure which country you are from however the Thai embassy in Aust will answer emails quickly. I could never get through on the phone. I’d suggest you keep your questions short and clear in dot point. Good luck.

And they wonder why traveler's are hesitant to go anywhere.

Book refundable tickets and hotels

Is it the STV you're applying for? Just book a no deposit completely cancelable accomodation on booking.com or similar. For the flights you can go to a site that does booking itineraries which are confirmed flights that lapse after a few days, will cost you about £15. I did both those things and had my visa approved within 48 hours

Just show a booking for first few nights

I have this same question as paying the full months in advance is not possible and not a great idea actually. What if you decide you don't like the accommodation or area an want to change? What if you want to stay a week or two in multiple locations to see the whole country? I was thinking to get an AirBnB for 1 week and find a place after walking around. Maybe even change cities. Would one week work for the application? Also.. because you'll need to do a border run, will your Covid and health insurance bought from the recommended options on the Thai Pass website reset so you'll need to repurchase during the border run? My personal insurance bought in Canada would reset if I leave the country and can't be renewed unless it's purchased while I'm in Canada

Why don’t you contact the Thai embassy in the country you live for advise.

Gee glad I didn't have to list my accommodation for the O-X visa :)

Come on a 60 day tourist visa extendable once for 30 days. Visit a neighbouring country and obtain a second 60 day tourist visa and extend that.

Which visa is for 6 months?

They won't reject your visa unless you can't provide the required documents.

What visa are you applying for?

As long as you qualify for the visa you are applying for, I haven't heard of anyone being rejected, and some embassies do want to see flight tickets and accommodations depending on the visa type. Each one gets to make their own rules.