Can I enter Thailand on a visa exempt entry and apply for a retirement visa based on my Thai Elite visa expiration?

If you have a German pension and get more income then baht per month you can also use a income statement from the German embassy in Bangkok.

"Merkblatt für die Ausstellung von Renten- und Einkommensbescheinigungen zur Vorlage bei der thailändischen Immigration "

you definitely need to get right over to your immigration office to see if they will require that the 800K baht you have banked here now can be used without showing it came from overseas. That is totally going to be their call. I know in Bangkok you have to show the 800K baht banked money came from abroad or you can't use it for proof of funds. I've actually seen people have to transfer it out and back in to meet that requirement 😕

okay first off you can't get in on a "visa on arrival" 😕 You're talking about coming in on a free 30 day entry stamp because of the nationality you are, and that's a visa exempt entry NOT a visa on arrival

With that being said

You can come in on a 30 day visa exempt and then go to your immigration office in Chiang Mai and apply for first the 90 day Non-O visa based on being over 50 (retirement) for 2000baht and then when that stamp runs down the year extension for 1900baht.

Keep in mind Chiang Mai needs a MINIMUM of 21 days left on your current entry stamp or extension of it to apply for the in country Non-O visa because of the under review period. SO that means when you get back in, GO to the immigration office get the hand out listing the requirements and apply before you run out of time on your entry stamp.

As long as you can show the money originated from abroad originally you should have no issues. BUT talk to them and see. Unless the german embassy here issues an affidavit of income from abroad notary letter you cannot show proof of funds in your account in Germany, it has to be here