The more people you have in a workshop, the more working in subgroups will be key.
A framework you can re-use
When the groups are pretty big and divergence is high, I tend to use this structure for my workshops:
Assign people to a subgroup: people get assigned to a subgroup. Either each subgroup works on the same challenge (for more depth), or each subgroup works on one part of a broader challenge (for more efficiency).
Solo brainstorming: before people go to work in their subgroup, they get some time to think on their own, on the task of their subgroup, and if they have an interest for the work of another subgroup, they can also jot down a few ideas.
Sub-group work: People share their ideas from their solo brainstorming and prepare a pitch they'll share in 5 minutes to all the other teams.
Pitching all together: Each group presents his results, feedback and improvement ideas get collected (for example ideas from the solo brainstorming from people who weren't in that group)
A shared canvas makes it easier
When people have to work in subgroups clear instructions help. To make them even clearer having them in writing and easily accessible helps a lot.
These days, I often build a big Miro board where each subgroup has a space for its work, with:
the instructions
and links to all the key documents that they need.
sometimes, I even add an example of what the final result of their group work should look like
Plan for extra synthesis time
When working with such large groups, especially when the ideas are very divergent, I recommend that you don't put too much pressure on making the perfect synthesis on the spot.
But instead, keep the workshop as a place where quick decisions can be taken, and some elements are left open for further reflection by a smaller team later.
Working with extreme amounts of people
When groups get extra big, you can use pitch and synthesis processes between subgroups.
For example, what gets shared with everyone in the end isn't the results of 30 subgroups but instead 10 times the results of 3 subgroups who merged their learnings together.