Hi MM as you have had no replies I will give you our pre covid experience, also we only ever booked onboard which was usually on the first sea day when they had a presentation of all excursions. It was either a full or half day excursion and you only received the actual departure time when you received your tickets, delivered to you cabin.
Experience would suggest that you do not rely on a return time as your coach can, and often is,be delayed which can be hours.
Good luck and enjoy.