to answer your question - no, you don't need a long term lease as a requirement for a visa. That being said, you do have to meet certain other requirements (age, financial, etc) to be approved for any of the multiple visas that everyone has mentioned.
1 year holiday visa? Not sure such a thing exists. There's the DTV (valid for 180 days but certain criteria must be met), retirement visa (90 days initially, extension for 1 year, again certain criteria - minimum age 50), multi-entry tourist visa (but won't allow you to stay for a whole year) So...you may want to research more
there's a Facebook group which is all about the DTV. You can probably get better, more accurate info from that group. The 'soft power' categories of the DTV are the ones with the most questions overall as the guidelines are vague at best.
The DTV sounds like a great option for you - 5 year multi entry visa with 180 days on each entry. Since you said girlfriend instead of wife, she would not qualify as a 'spouse'. She can sign up for Muay Thai classes or cooking classes under the 'soft power' option.
Ann is 100 percent correct. As your children (and even grandchildren) get older, you will realize they have their own lives. It's not selfish to do what's best for you. We left the states almost 4 years ago and don't regret it for 1 second. We go back to visit every so often (every 12-18 months, but it's so expensive 😔). Check out as many different places in Thailand and then make the move!
Vietnam has 2 Thai embassies - Hanoi and Ho Chi Minh City. Any correspondence from any country's embassy should come directly from that embassy and not Thailand.
the medical certificate is only for the Non-OA. Non-O doesn't require a medical certificate or insurance. There are definitely differences between the 2. I'm not an "expert" but typically the Non-O is easier.