I provided one cover letter detailing what I submitted and my intent of wanting the DTV. Portfolio I included CV, emails of appreciation from clients, sample of work, company guidelines for remote work, company website info.
I'm similar to you in in terms of freelance contract. I provided the business registration of the company I work for and contract of employment, bank statements highlighting payments from the company, letter detailing I can work for them from Thailand for the 5 year duration of the DTV.