You know what? Your idea is good. Furthermore, why don't you spend few days in Nabucco, then stop over a couple of days in Sangalaki and end your trip in Derawan? You can arrange everything through one guy: Sastrodiantoro Senoadji (Pak Seno), Email
senomentik@indo.net.id. He was my instructor, and deals regularly with all these resorts. He will take care of all transfers, bookings, pick ups, etc... all in one package. And by the way, he will not give me anything back for the referral.
Why would you do that? because by staying a few days in each place, you will increase your chances to get the best of each, as you mention. The big fishes in Nabucco, the mantas in Sangalaki, and whatever there is in Derawan (I have not been there).
Nabucco is 3hours 1/2 away from Berau (town where you will land wherever you stay). You will be happy to do it on your way in, but on the way out, it is lengthy, especially at 5 AM... Nabucco is a great place, good and friendly management, but only 2 AC bungalow. No beach, no swimming pool, but splendid view from your balcony, just over the water. Porfessionals.
Sangalaki is way below in terms of standards. But all bungalows have AC and you are on a " desert" island, with the white sand, the crystal clear water, and the turtles nesting. This is unique! And for the cockroaches, be realistic, there are everywhere in Indonesia! And 2 dives only with mantas is not enough. You will want more.
Derawan, I don't know, but others seem to have liked the place.
Regarding the sea, it can be rough at that time of the year. And this year, the rainy season is a real one.
Anyway, wherever tou stay, you will have splendid dives. Lucky you!