Storing tree data in a database for fast reading of the whole hierarchy

Question

I have a class that looks something like this:

class Entity
{
    public string Id { get; set; }
    public List<Entity> Children { get; set; }
    
    public void DoStuff(Data data)
    {
        // stuff
        
        foreach (var child in Children)
        {
            child.DoStuff(data);
        }
    }
}

And I need to persist this in a database. Available options: MongoDB, SQL server or PostgreSQL.

I was wondering what is the best approach for storing this kind of tree data considering these conditions:

• Read performance is most important when the whole tree is queried - parent with all of its descendants.
• Usually only one child will be present per Entity in the Children list. But the depth of the tree could be 50-100.
• Once the parent is created I need to be able to add/delete children. But the speed of this operation is not that important.

In JSON data looks like this:

{
    "id": "xyz_0",
    "children": [
        {
            "id": "xyz_1",
            "children": [
                {
                    "id": "xyz_2",
                    "children": [

                    ]
                },
                {
                    "id": "xyz_3",
                    "children": [

                    ]
                }
            ]
        }
    ]
}

At the moment I'm using MongoDB and storing a single tree per JSON document. And the application code looks something like this:

var entity = mongoCollection
    .Find(Builders<Entity>
        .Filter
        .Eq(o => o.Id, parentId))
    .SingleOrDefault();

foreach (var data in GetData(10000))
{
    entity.DoStuff(data);
}

But reading a couple of previous posts here and here mentions that it's not such a great idea to store big documents. So I'm looking for alternatives.