How can I and my future wife qualify for a retirement visa in Thailand if I am over 50 and she is under 50?

Once married - rules are different. She can attach to your retirement non o visa, as a dependent. I’m almost certain the 800k required doesn’t apply for her.

Exactly as

@Bra****

Thurkettle has explained. I will add that you will need to be legally married before you start the process

Your wife can apply for a "trailing spouse" visa as a dependent under your visa. But this is not possible to do in Thailand. You would need to apply for a non-O visa based on retirement from a Thai embassy in your home country, and then your wife would need to apply for a non-O visa based on being a trailing spouse to your visa.

Then once you arrived in Thailand together you will both be stamped in for 90 days. You would need to deposit the 800,000 in a Thai bank account very quickly, because it needs to be in your account for 2 full months before you can apply for your 1-year extension.

Then you apply for your 1-year extension and only after you have your full 1-year extension can your wife then apply for her 1-year extension based on your extension She will not receive a full year but will receive dates that match the end of your extension.

The reason you need to get the visas before you travel to Thailand is because your wife cannot get a non-O visa as a trailing spouse in Thailand, nor can she get it at any Thai embassy in SE Asia. The last embassy that still offered this has ceased doing so as of this month.

If you are not married, it is exactly the same as you doing it yourself, except no retirement qualification. If you are married then a spouse extension will be available.