im Dutch and business owner. I got it so easy with kvk proof. I can show u exactly how i did it and got mine in 5 days. Send me a pm if u like. Can share my outline and letter.
you have a business registration? You can ask 2-3 clients if its ok to share the agreement you have with them. Also emails between you and the clients showing they are your actual clients. Describe your business with them from A to Z with examples and proof. Add some paid invoices as well.
I have my own company (sole proprietorship) in my home country and we do have a DTA with Thailand yes. I also do not want everybody to know back here that I stay Thailand > 6 months because of some other stupid rules. So again, if I pay my tax here back home lets keep it this way, I don't mind, but double tax is ridiculous.
but what if I pay all my taxes in my home country every year and my home country also have a Double Tax Agreement with Thailand? This part is not clear for me yet.
I am a business owner and I only showed my business bank account. Besides, I showed proof I am the owner of the business. This because only my business name is showing on the bank statement. In the Netherlands this can be done with an extract from the Chamber of Commerce.