I don't know if the Singapore Embassy in Thailand issues an affidavit or not. If you don't know already, it might be worth checking.
You can refer to this for requirements in-country Non-O based on being over 50 to get some idea of how to prove your finance.
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/9.FOR-RETIREMENT-PURPOSES-50-YEARS-OLD-NON-O.pdf
And attached is a rough comparison for non-OA and Non-O (retirement). You need to check the detail for those to make your own decision.