Some thoughts.
He easily could be Portuguese from looking at him, especially Cape Verdean. I would guess the name was de Souza or similar at one time. Was he really born "Frank" and his father "Jack"?

or did he Anglicize everything, including his parents' names?
Have you looked for his parents in Australia? Frank was only 17 when he arrived, so he could have been on his own, but maybe not. I don't know how many Australian records are on my subscription to Ancestry, but I'm not finding much.
He named a son Albert, so maybe there is some truth to his having a brother Albert. Also, are the children's names from Mary Pearsall's side of the family? If not, some of them could be from his. That might help.
---
Here is an Antonio de Souza, born 1843 Azores, said to have married 1871, Portland Estate, SA, to Caroline Hill. Sons James, John, Albert, etc. Said to be a mariner in 1877. Died working on the wharves.
https://www.ancestry.com/family-tree/person/tree/8634782/person/-918440764/factsI think Souza in Portuguese is like "Smith" in English, so they don't have to be related, but was there a Portuguese community in Portland Estate? Worth looking into.
-----
"Mariner" and "wharves" is the track I was on for Frank Peter Souz's family as well. I have a feeling they were in Seattle long enough to have Frank, and could have been 1000 miles away the year before. The records are going to be tough.
And were his parents married, or was Mary Ann Jane a native of Seattle and his father a sailor? I haven't looked for her yet.
The DNA results could help a lot. Keep us posted.