Have you tried submitting your Annual Tax Return, Bank Statement, and profit and loss statements?  I read from one of the articles online that these 3 Types of documents can be used as proof of income.  You can read more here: https://bungalow.com/articles/how-to-show-proof-of-income-when-youre-self-employed

It is really hard to produce proof of income when one is a freelancer or self-employed, you can also inquire if they will accept the following document to prove your source of funds before gathering and preparing those documents.

You can also check this article to have more ideas: https://lili.co/blog/proof-of-income-self-employed

Always inquire first before doing anything so that you won't waste any effort while preparing these documents.