How to limit the selection of the first level block in Gutenberg editor

Question

Create a new custom block, lets call it CustomSecondary, that contains an InnerBlocks instance.

When registering this block set it’s parent property to your main custom block and

registerBlockType(
    'custom-secondary',
    {
        parent: ['your-main-block],
        edit: () => ( <InnerBlocks />)
        { .. your other properties }
    });

This will make it so this block can only be inserted into your main block.

In the edit property of your main block add the allowedBlocks props to your InnerBlocks instance and only allow your custom-secondary block

<InnerBlocks allowedBlocks={[ 'custom-secondary' ]} />

Now when you insert, you’ll only have that option and then the custom-secondary can have any blocks you want inside.

To control the template for the post it self, you can add the following filter:

function myplugin_register_template() {
    $post_type_object = get_post_type_object( 'post' );
    $post_type_object->template = array(
        array( 'core/image' ),
    );
}
add_action( 'init', 'myplugin_register_template' );

You might want to consider using locked template with a single block that can have items inserted into it.