If I kept my pension in my home country and do not send it over to Thailand, but live off the wealth I earned before 2024, do I still have to tax the pension in Thailand?

Savings in the bank from pre2024 are not a taxable source of income in Thailand. Also you do not have to declare a pension if it is not remitted into Thailand. However, if you do remit that pension, then it becomes a potentially taxable source.