reason to get the visa is if you already have a recent history of a lot of visa exemptions, and it seems you're trying to live in Thailand off them, as many immigration officers have been "cracking down" more recently
Write a cover letter with the explanation you just gave regarding your business and self-employed status, and attach 1-2 sample contracts and perhaps invoices. If concerned about confidentiality, can redact client names. Also consider attaching business registration document as this seems to be asked for. Compile all as a single pdf to submit as only one document can be uploaded per question (at least during initial application).
each question allows one document to be uploaded, so if you want to submit multiple documents (for a question) you'll need to merge them into one, or submit one and wait for the embassy to request further documents
Upload the biodata page of passport where it asks for that. Upload location documents (including visa stamp in passport) where it asks for proof of location.