Hello, Helen; nice to hear from you (I was starting to wonder if anyone was still interested).
Yes, William and Keziah (my great-great-grandparents) are your ancestors too. Elizabeth Powe (one of the children who came out from Britain on the 'Racer') married John Miller in 1873, not long before William married for the second time.
Keziah was one of the first to be buried in Haslem's Creek cemetery (now Rookwood Necropolis), in October 1869. William is buried in Waverley Cemetery in Sydney's eastern suburbs (see attached photo).
I have a number of other documents and photos, including one of William, but they have not yet been digitised.
None of the certificates I have acquired over the last 25 years identifies him as William JOHN, but this may be the case (he would not have been John William, as his older brother was John Score Powe).
Feel free to contact me directly at firstname.lastname@example.org
if you wish.
PS: Let me know if there is anything you'd like scanned (BDM certificates etc).