Go big or go home!
Especially with eight...four couples. Us guys are OK with just two heads, but the ladies like the idea of en suite heads. They really like the electric heads too!

You didn't mention gensets and A/C, but that is another advantage of bigger is better. The size with MooringS you mention aren't AC.

Lastly, the TUI R&C cats are fantastic at the helm. But you are cruising, not racing!

remember, on a charter, you are likely 4 to 6 hours maximum on the helm. You'll spend far more time on the deck, cockpit, salon, cabins, etc.