Pay attention to your model's instructions before taking action

Once a views el is connected to the DOM, rendering will immediately attach the View to the DOM, resulting in the top level view being attached right away. This means that if callbacks are placed in onRender, they will be attached sequentially leading to reflow.

An alternative is to place subview shows in onBeforeShow, creating a tree of Views that will be displayed at once, assuming the top View has been shown using show. The question then arises as to where to call the top level show. Two options are possible:

The straightforward option

The simplest approach is to have the top level LayoutView display its children in onRender, with subsequent child views showing their children in onBeforeShow. This may result in a few paints (one for the top level and one for each region it displays), but not an excessive cascade and likely no performance concerns (especially if there are no visible performance issues yet).

The slightly more complex option

If a single repaint on model change is desired, a wrapping View can be created as indicated in the question. Instead of having the child view directly reference it, ensure the wrapping View holds a reference to the model and triggers a layout refresh when the model changes. In this case, all callbacks can be onBeforeShow, and changes will lead to only one paint. There will be no dependencies between Views, ensuring consistent display through the same callback.

var LayoutContainer = LayoutView.extend({
    initialize: function() {
        this.listenTo(this.model,'change',this.reshowLayout);
    },

    reshowLayout: function() {
        var layout = new Layout({
            model: this.model,
            container: this.containerRegion
        });
        this.containerRegion.show(layout);
    }
});

Marionette ensures proper DOM insertion

Marionette follows a specific process for attaching children of a Collection/CompositeView to the DOM only after everything has been rendered, including parent and child nodes. The sequence of actions can be observed as follows:

During the execution of Region.show():

show: function () {
  // ...
  if (_shouldShowView) {
    view.render();
    // ...
    this.attachHtml(view);
  }      
  // ...
}

Upon calling region.show(), view.render() is triggered, followed by CompositeView.render(),

render: function() {
  //...
  this._renderRoot();
  this._renderChildren();
  //...
  return this;
}

The functions view_renderRoot() and view._renderChildren() are invoked in sequence within this method. It's important to note that these functions do not directly attach the views to the DOM; they simply prepare the templates and store the HTML nodes in memory.

Final Step - DOM Insertion

Subsequently, in Regions.show(), the actual insertion of the views into the DOM occurs through region.attachHtml():

attachHtml: function(view) {
  // empty the node and append new view
  this.el.innerHTML='';
  this.el.appendChild(view.el);
},

If you opt not to use the region manager and manually call .render() on your view, remember to insert the view into the DOM similar to how region.attachHtml() does it.

All Children Linked to Parent Prior to DOM Inclusion

In a CompositeView, the rendered children are attached to the parent view either through CompositeView.attachBuffer() (for initial collection render or upon collection.reset()) or via CompositeView._insertBefore/_insertAfter (sortable/unsortable children).

All these methods ultimately involve

collectionView.$el.append(childView.el)

In cases where the parent view is not yet part of the DOM, all attachments are made to a collectionView.$el existing solely in memory.

Simplified Event Handling

The design of Marionette simplifies the rendering and injection of children views by removing the need to manually handle when each child should be rendered. Proper nesting setup from the start ensures a recursive process where all children are linked before being appended to the DOM. If you require guidance on setting up nested views effectively, share your relevant code for further assistance.

Revamp LayoutView.render()

After carefully considering the feedback and clarifying the issue, I have come up with a potential solution:

Instead of simply calling ItemView.render() within LayoutView.render(), I recommend overriding the former in your parent LayoutView. Here is how you can do it:

render: function() {
  this._ensureViewIsIntact();

  if (this._firstRender) {
    // Exclude resetting regions on first render
    this._firstRender = false;
  } else {
    // On subsequent renders, re-initialize `el` for each region
    this._reInitializeRegions();
  }

  var layout = Marionette.ItemView.prototype.render.apply(this, arguments);

  // Render and attach the child after parent rendering
  this.someRegion.show(new view(new model()));

  return layout;

},

I must admit that I haven't tested this approach before, but sometimes thinking outside the box is what leads to successful solutions.