As Max has pointed out, it's one or the other. I don't like to tie up 800k effectively for the rest of my life, so I use the "income" method of 65k/month transfers, as this money can be used as soon as it registers in your bank account
You might be overthinking the whole thing. I did it yesterday at CW. Didn't need an agent. Just get your documents in order and it takes 40 mins and 1900 baht. So easy!
As Brendan has said, although the O, OA and OX are colloquially referred to as "retirement visas", there is no requirement at all to be retired, only that the applicant is over 50. As with most visas in Thailand, employment in Thailand is prohibited. Many FIFO workers use the "retirement" visa as it's more convenient when making multiple trips to Thailand (where many have homes, wives, girlfriends, boyfriends), and the retirement visa means they don't ever get hassled by immigration when entering