What should I do if my contracting company doesn't provide official documents for my visa application in Thailand?

The company I've been exclusively contracting for the past five years doesn't use/have a letterhead, they're also not very responsive to getting the business registration document either. It's a Japanese company - my boss is willing to sign a letter but without the letterhead it looks so unofficial.

Any advice? Is my 'employment' document looking a bit weak? I don't have payslips either. I have contracts, invoices, and receive a monthly bank transfer.

The ironic thing is I've never even lived in Japan and have been working remotely the entire time already. The idea of asking for permission to do something I've already been doing for half a decade is kind of crazy.