Can I apply for a retirement extension if I entered Thailand on a non-immigrant O visa based on marriage?

Non immi O based on marriage I believe you only need 400K in bank. If they’re saying you need 800K your visa most likely nothing to do with marriage.

Have you been legally married in the amphur

Check your used visa sticker.. Your sticker may or may not have the reason of Non-O. At some immigration office, they let you apply for the 1st extension with only the reason written on sticker. .. At least before COVID situation. It's Better to check your sticker then your immigration office.

If nothing related to what your Non-O based on written anywhere in your visa sticker, you would be okay to apply based on being over 50.

I believe you can... The basic 90 day non imm O is used to obtain a retirement extension. Just Deposit your 800,000 in a Thai bank account in your name only as soon as possible to start the 2 month Seasoning / aging of the account.

The TM.7 Application for for the Extension Based on Retirement is easy enough. You must make copies of every page of your passport. Have a rental agreement, with copy of the house owner's ID card and Tambian Ban (deed) Plus a map to your house. Bank Records, updated Bank Book. And a Bank letter and bank statement. All have to be very recent (day before) The bank letter basically states that the account is actually yours.

> Is it possible for me to get a retirement extension?

You can get EITHER an Extension of Stay based on Retirement OR an Extension of Stay based on Marriage to a Thai Female National.

>When is the best time to apply?

It depends on the immigration office. Some immigration offices allow you to apply NO SOONER than 30 days before your current entry stamp expires, and others let you apply NO SOONER than 45 days.