They will ask you for any docs they need. A portfolio is very open ended it can include a resume, sample of work, client letter/emails of appreciation etc
Yes you can have the funds in a non Thai account. There is no minimum income requirement. I was approved on a low income as there was a delay with my main online contract being renewed. I did submit additional savings to show financial independence. Showing additional savings will help.
You need to be in a remote position that allows you to be in Thailand working online. Sounds like the best option is Softpower via a course such as MuayThai or cooking or medical treatment