In Rurutu, I went out with Tareparepa. My contact there was Teiti Tavita. try
Teiti.tavita@gmail.com. If you do Facebook, you may be able to find them there.
You may not be late. I went in late September 2022. I actually prebooked with another operator, but upon arrival, that other operator was a no-show. So Teiti stepped in and he was able to accommodate us (a group of 5) at the last minute. They have multiple boats.
I haven't done Marquesas.
As to what to do after Rurutu, FWIW, when I went in 2022, I did Rurutu followed by more whales in Tahiti and then diving in Rangiroa in early October.
Before you decide which to add to Rurutu, you may want to check availability of Air Tahiti flights on the day you return from Rurutu to Papeete. I think that there are daily flights from Papeete to Rangiroa and to Tikehau but not so to Fakarava.
I was just in Fakarava, Rangiroa and Tikehau Dec. 2-20. There are sharks in all of them.
In Fakarava, there were lots of sharks in the passes, north and south - mostly grey reefs and a few white tips and reef black tips - and they were more eye-level. A couple of silvertips as well. And there were a couple of manta cleaning stations on the outer reef dive in North Fakarava. No dolphins.
In Rangiroa, the sharks in the pass or at the mouth of the pass were deeper, and we also saw from above one tiger and one Great Hammer down there in the 150-200 ft. area. Also one big silvertip down there as well. Didn't see any mantas but did see dolphins.
In Tikehau, didn't see a single grey reef or silvertip, just a few whte tips and two black tips. But in 9 dives, saw 4 Great Hammers and 5 Tigers, and in 30-60 ft depth. But as I said in another post, for me, it gets a bit trite and tiring swimming all over when there are no Tigers or Great Hammers. There is a good variety of fish, but they are the same fish day after day. Also, there appears to be more Great Hammer activity December-February. Don't know in the September timeframe. Tkehau also has a manta dive at a cleanin gstation but it is in the lagoon and the viz is bleah.