I went to a visa agent in Chiang Mai in 2023 after getting sick of walking around banks and queuing up to be refused an account. One bank I didn't try and have been told since it's possible is the Bangkok Bank at Airport Plaza. I ended up paying a visa agents I think 1500 baht which included the initial bank deposit. It could have been more but not over 2000 baht I just don't remember the exact amount but I was surprised how cheap it was.

I then deposited the 800,000 baht and applied for the type O visa, then 3 months later had it converted to an extension of stay retirement visa. This was all done by Napa Visa in Chiang Mai. How I did it was coming in on a tourist visa exempt, went straight to the visa agent the next day and they started the process, first getting me a 30 day extension on my visa exempt to give me 60 days to get the type O and bank sorted. I think you need the 800,000 in the bank for 3 months to extend to the retirement visa not to get the type O. But I'm not 100% on that one.