We recently got back from B2B (Transpacific and then Best of Japan) on the Millennium. The ship for the most part is well maintained and we enjoyed the size of the ship. Completely different experiences on each leg as you can imagine.
Regarding the OPs question about things to do on the 4 full sea days, I would say yes. The cruise director, Patrick and the activities team were very good. Of course there seemed to be less planned activities on the sea days for 2nd leg compared to TP, but there was still plenty to keep guests busy or to relax as per preferences. The service was excellent for the whole cruise, the food in BLU was outstanding and the Celebrity and guest entertainers were awesome.
I‘m hoping to write a review and post shortly since I realize it‘s been a long time since Celebrity was last in Japan, but in the meantime, I‘m happy to answer any specific questions.