Taxability

This is dependent on which airline is used. If booked through a Thai airline and paid through a Thai office, then this is remittance of foreign-sourced income and is taxable.

Credit cards and debit cards are quite a grey area, as they are not specified in the Thai income tax law. Under audit, if it is seen that these are being used for day-to-day expenses and living, then credit card usage is likely to be classed as remittance. If you use your debit card, it depends on the source of the funds in the debit account. If it is classified as foreign-sourced income, then it is assessable income.

It depends on the source of the funds. If it is from taxable income when you were a Thai tax resident then this is classed as remittance