ChatGPT will sort that out a employment contract/certificate. Professional portfolio is open-ended, you can inc resume, samples of work, client references,
Applying in the US, you should be ok with no monthly income evidence. Apply with a 6 month plus course. Many US embassies are fine with 3 months seasoning of the 500k THB, not heard one needing 6 months.
Where are you applying? Very few embassies require 500k THB to be seasoned for 6 months. 3 months is most popular. Showing monthly income for Softpower depends on the embassy Taipei requires it.