I am 70 and just got my retirement visa in September. If applying in Thailand for non-O visa then no health required. I used an agent and it took 30 days for approval. No need to show income if you have the 800k in Thai bank. No resolution on income tax yet. Like you I pay taxes in US so treaty may help but...
Received my retirement visa last year. Made 1 trip (30 days) and opened a bank account with 800k Baht. Returned 2 months later with 60 day visa and applied for retirement visa. Had to show airline return ticket to US to board so booked a cheap flight to Singapore (60USD). Took 30 days for approval.
I am in the process of moving from to Florida. Went on a trip and opened a bank account through agent. Returned home and deposited the 800k in the account. Went back to Thailand 3 months later and applied for retirement visa through agent. Took 30 days and was approved. Had to return to US for routine medical appointments. Will return to Thailand in February for the long term. BTW , I FOUND Korean Air to be the most comfortable and one of the shortest flights (26-27 hours with transfers. Fly to Atlanta the Korea then Bangkok.