When I did similar trip I looked at each segment separately to find space - I couldn't find any OZ back to US so did Star partners via Europe home with a stopover. My OZ flight out of ORD had lots of space but old J angled seats, the JFK, SFO LAX new seats never had space when I searched for a two month period. I did PHL -ORD on US, ORD-ICN on OZ then ICN-SIN and SIN-MEL on SQ paid cash internal MEL-SYD--Hobart-MEL then award tix continued MEL-BKK and BKK-CDG on Thai including A380 with final LH leg canceled so US back to PHL -