Well...Suva is basically one of the very few places that can hold infinite amount supplies & fuel when build up. New Caledonia of course too, but then it's same as taking it all the way to Sydney anyway. PagoPago might work too. Of course you need to heavily garrison those places anyway against invasion.
Sardaukar, I disagree here.
In my opinion Suva is actually one of the worst possible positions for a staging post as it usually is on the Japanese nice-to-have list, exactly because of its
As aphrochine said, the area ia full of bases which can be built up for the job, the most obvious one (and also chosen in my last PBEM) is Vavau. Bigger harbour than Suva
if you build it up, in case you need to. But it is by far not the only one: Lifuka, Tongatapu, and further south a few places in the Cook Islands, last but not
least, Tahiti, if you really have to deviate far South because of a Japanese attack.
I reccommend building up an efficient and a alternate route, where the alternate can be used in case the efficient one is threatened. When the fronts stabilize the routes
obviousely move a bit further North, maybe along the Phoenix/Ellis Islands and later the Gilberts and Marshalls, if this is still neccesary by then.
Well..you need base that can be build to 10 combined (airfield and port combined, 5+5 as average) to make it really viable supply hub. Of course Suva is high on IJ list for obvious reasons, and similarly Allies should make big effort to retain it.
There are alternatives, as you and others have stated, and they work well too. Just that losing Suva & New Caledonia forces Allies to have fairly limited "Central Pacific strategy". Suva is both excellent supply dump and staging area for future advance.
"To meaningless French Idealism, Liberty, Fraternity and Equality...we answer with German Realism, Infantry, Cavalry and Artillery" -Prince von Bülov, 1870-