You don’t have to have a big grooming session to analyse all the user stories for next sprint. That will be tiresome since it takes long, and people won’t like it.
Instead you can break them into small ones, like a one-hour session everyday for a few days while you are still going on current sprint. During every session, you still try to clarify the user story as in detail as possible and work out a design and task break down on spot if possible.