Hi Jordi,
Just saw this as I registered recently. Diving in Nosy Be is fantastic! I can really recommend Scuba Nosy Be, Ocean Dream (did the Mitsio Archipelago tour with them, also great) and Diving Cattitude (did not go with them myself because timing did not match but only heard great recommendations for their croisières).
I did not make it to Gregs Wall yet - had planned for end of October (when one might see whale sharks and hump back whales) to do another few days with Scuba Nosy Be again just off Nosy Be. They do many great sites (I especially love Atnam, huge gorgones and leopard shark followed us all along the wall ) , and a really great club. They usually do double tanks and one is back for lunch so enough time also to explore the island. They also do day tours to some of the other smaller islands around where you take seafood lunch). And they just got a boat as well, going to Radames Archipelago (where Gregs wall is)- they have a little different concept than the others where you sleep on the island, either in a nice hotel or you can camp on the beach.
Ocean Dream and Diving Cattitude both offer Mitsio and Radames Archipelago tours. Mitsio is more shallow though there are a few 40m dives and Radames most interesting dives are deeper (which is why we waited as our youngest is just 15 and only just got his SSI AOW deep).
That fell through for us due to covid though. Nosy Be was closed for tourists in May due to covid when you wrote and it might still be, not sure. When did / do you plan to go? If you look for flights look at Ethiopian Airlines - of course with covid all changes but they had great flights from many places with change in Addis directly to Nosy Be. Might be cheaper and faster than via Tana as inland flights in Madagascar are quite expensive.
Good luck!
