I found that my great-grandfather filed a declaration of intent AND a petition to naturalize (see here: https://www.familysearch.org/ark:/61903 ... AZK2T-69PZ) prior to the birth of my grandfather.

My grandfather was the youngest in his family and was born in June 1928. It appears this application was made when my grandfather's 4 siblings were alive, but not my grandfather, in October 1925.
Does this mean my jure sanguinis path is likely over? Ugh.