Anonymous participant well, they can ask you for everything, including your pics, your individual tax reports etc. But they require it only if you're not convincing for them. If you have a legal business, you don't have to worry. Be sincere and explain everything to them, they will understand.
There is no requirement to show tax proof. You need to prove that you legally work abroad, you don't work for Thai companies, and you are allowed to work remotely in Thailand.