Can I apply for a One Year Retirement O Visa after entering Thailand with a 90-day Visa?

Contact the German embassy in Thailand if they can issue an income certificate so you don't need to keep money in a Thai bank.

You can certainly apply for a year extension based on retirement if you came in to the country on a 90 day non-O visa you got from the thai consulate before you come here

These are the things that don't matter concerning your retirement extension; working here 23 years, owning property, paying into the thai social security system, having a thai wife or children 😮

You're applying for a year extension of stay based on being over 50 using the banked money method by showing 800K baht in a thai bank account in your name only for 2 months before you apply. The funds will have to stay in the account for 3 months after you get your yearly extension and then the balance can't go below 400K baht the rest of the year.

The year extension is 1900baht, and at most offices you can apply for the year extension when you have 30 days or less left on your 90 day stamp.

After you're here GO to your immigration office and get the hand out listing the requirements specific for your office for the retirement extension before you need to go apply so you can get all the documentation together and get it done in one attempt