If you have your own company that contracts to the industry just produce a work contract to yourself, produce an employment certificate that authorizes you to work remotely in Thailand, you're all set. If your employer is willing to write the certificate then that's fine too. Your 500K will need to be in a personal bank account, not your business account. Embassies will likely ask to see your tax return and six months of bank statements where you can highlight the payslips/income etc. There is no income requirements tho, and they just want to see that you aren't a fly by night entity. Plenty of plenty of past posts on successful applications with details for you to glean how to smartly present your application, use the group search tool with keywords. You can write a very brief cover letter outlining your work remotely as a freelancer. I wouldn't include too much detail, and upload that as part of your PDF application.