Your question is nonsensical, why would you pay tax after retirement? What amount of income triggers taxes? What is alot compared to a moderate amount? Please be more precise to get a relevant answer to your question.
No you don’t have to inform anyone that you don’t pay tax in Thailand. Please however be aware of the Singaporean tax residency rules, you are fine for 2025 but even though you are Singaporean you will lose tax residency if you don’t spend enough time(183 days) in Singapore in a tax year. Obviously if you are maxing out the DTV you lose the protection of the DTA in 2026 when Thai tax residency will apply and you may be required if you qualify to file Thai personal income tax.