You need a forward staging base. Try to haul the first wave all the way from Pearl and they'll unload on the beach with 50+ disruption. You need someplace big where they can unlaod, rest, and re-load for the landing.
Where, though? You need a base or two that can hold 30000+ troops. Eniwetok has a limit of 6000. No easy answer.
Mariannas, I agree, is a real challenge. I have never gotten far enough to invade, but they are a bear, and the Japanese have plenty of time to fortify.
I also think taking one of the 4 smaller islands (Roti, Antahan, Almagan, etc) has to be on the list. This way, you can dock an AKE there, and AOs, and have a stantionary airbase. Something you can use to sustain Naval Ops for a month. You need a piece of ground somewhere to do that, and definitely land closer than Pearl. They also all have a stacking limit of 35K or so, enough to rest and refit some guys close to the targets.
Marcus might be a better prelim than Eniwetok; it can't be defended with other than CVs, and taking it rips a big hole in the air search.
I don't like going as deep as Marcus. Personal style.
When I played Marianas with the AI I used Eniwetok for supplies, fuel and AD/AR. I unloaded assault troops for a few days, ate the overstack (I had supply dumps nearly seven figures), reloaded the first wave in Amphib (fairly slowly, but I had naval support too) and sent them in. The second and third waves I left on the ships from Pearl and let them recover disruption on the beach.
If you have time to take Kwajalein and Roi Namur that helps a great deal too with re-load rates. I also pushes his subs back to the inner perimeter for basing.