Can I apply for a Retirement-O visa extension in Thailand with monthly deposits of ฿65,000 instead of 800k baht upfront?

No they want the 800k for your first non-o

No you can’t use monthly transfers as your initial application for a non O visa in country. You can transition to those provided you meet the initial criteria of the 800k for the first visa application and then leaving it three months after the first application for an extension and not going below 400k for the rest of the year.

If you regularly deposit 65k baht and don't touch it after 13 months you will have amassed the 800k required to apply for a Non-O visa based on being over 50 and also have the 800k required for the initial extension. That extension requires you to have 800k in your bank 2 months before applying for it, keep it there for 3 months from the date of your stamp and never let it fall below 400k for the remainder of the year. If you then continue to send 65k baht every month you can use that to apply for the second extension. Once you obtain the second extension the 400k can be removed.

No. First time they want to see the 800k dosh in the Thai bank account. Future extensions after one year are fine