Different drag policy for different model indexes in QAbstractItemModel

Question

I have a tree model that contains groups, which can contain items and nested groups. All groups are visible in the root of the model, but they can reappear as group items as well.

I would like to allow following operations:

That means:

• Groups can only be dropped inside groups, not on the root element
• Items withing groups cannot be moved to other groups or the root element. They can only be moved inside that group (up and down).
• (this works) The root element accepts items from another view (not shown here)

Currently, this is how I set flags for items:

Qt::ItemFlags ImportGroupListModel::flags(const QModelIndex & index) const
{
    Qt::ItemFlags flags = Qt::NoItemFlags;
    if (index.isValid()) {
        ImportGroupModelItem* item = static_cast<ImportGroupModelItem*>(index.internalPointer());
        if (item != nullptr) {
            if (ITEM IS ROOT ELEMENT) {
                flags = flags | Qt::ItemFlag::ItemIsDropEnabled | Qt::ItemFlag::ItemIsEnabled;
            }
            if (ITEM IS TOP GROUP)
                flags = flags | Qt::ItemIsDragEnabled;
            if (ITEM IS TOP OR NESTED GROUP) {
                flags = flags | Qt::ItemFlag::ItemIsDropEnabled | Qt::ItemFlag::ItemIsEnabled | Qt::ItemFlag::ItemIsSelectable;
            }
            if(ITEM IS ENTRY) {
                flags = flags | Qt::ItemFlag::ItemIsEnabled | Qt::ItemFlag::ItemIsSelectable;
            }
        }
    }
    return flags;
}

So basically what I want is to allow internal move only for any model index that is not the root index. How to do it?