Let me give a more practical example:
This might be the type of users DeltaChat perhaps is catering to. (Not trying to comment on their activities, just what’s the typical DC audience.)
Would they have likely picked a moderated group at that size of movement?
I would say it’s very likely.
Would it have been unproblematic for them to use a shared permanent symmetric key?
In my opinion no, since their recorded encrypted data streams will likely be looked at now, if there were recorded before.
Would it have been essential to them to have shared history on join?
In my opinion no, maybe it would have been convenient, but likely they were manually observing new joiners to constantly re-evaluate trust where no shared history seems safer.
IMO, therefore a DC group with such properties isn’t a good fit for DC.
Edit: especially since I think many of the features are in theory doable basing this on private groups, too. E.g. by the core auto dropping messages from strangers if they got introduced via groups marked in some way, making the core not accept being added to some rooms based on a marker, etc. And 400+ people seem to work, too, since we tested this once.