RESOLVED: OK, I think it’s clear now, and I understand where I went wrong. The Non-O visa is available through the Thai eVisa website, but it’s not labeled as “Non-O”...it only shows up if you select the option to “stay in Thailand for no longer than 90 days” under “Retirement.” I had ruled that out because I want to stay longer, and I didn’t realize that you could apply for the correct 90-day non-O visa first, then extend it inside Thailand.

Since I didn’t see a way to apply for Non-O in advance and didn’t want to buy extra health insurance (I already have excellent coverage), I was planning to enter as a tourist and adjust status to non-O. Thanks to your guidance, I now see exactly how to apply for the Non-O in advance and skip that workaround.

Really appreciate everyone’s help. This was very useful! 🙏

“…stay in Thailand for no longer than 90 days”? This is what’s so confusing. Obviously I want to stay for longer than 90 days, so I ruled that one out immediately.

@Marianne *******

Thanks! Which of these is the non-O visa?

EDITED: I'd enter as a tourist because I have excellent health insurance as a federal retiree, but I understand that the documentation is not sufficient to apply for an O visa online, whereas it's not a requirement to show my U.S. health insurance when converting in-country. If any U.S. federal retirees with FEHB coverage have managed to get around this problem, I'm all ears! Thanks to all commenters for the advice!