2 days after execution of the transaction

T-Bills, Central Bank Bonds minimum Bt1mn
Government Bond : Bt10 million
Corporate Bond depends on placement
PP or Private Placement at least Bt 10 million
PO or Public Offering any amount

Yes. If you are an individual person, your interest income and capital gain are withholding taxed by 15 percent.
No. If you are a juristic person, you pay corporate tax 30% at the end of the fiscal year.