Replace block page template dynamically using PHP

wp_replace_block_page_template.php

<?php

/**
 * Before the block template is displayed, replace it with a different block template.
 *
 * IMPORTANT: You must create the block template through the editor: Templates > Add New
 *
 * @param string $template
 *
 * @return string
 */
function rs_replace_create_post_block_template( $template ) {
  
  // Replace this conditional with your own logic to target a specific page or url:
  if ( get_query_var('my_action') === 'create_post' ) {
    
    global $_wp_current_template_id, $_wp_current_template_content;

    // Use a block template from the theme
    $block_template_slug = 'wp-custom-template-crm-create-post';

    // Get the block template post in the database
    $post = get_page_by_path( $block_template_slug, OBJECT, 'wp_template' );

    if ( $post ) {
      $_wp_current_template_id = wp_get_theme()->stylesheet . '//' . $block_template_slug;
      $_wp_current_template_content = $post->post_content;
    }
    
    // Note that block themes do not replace $template. If we did that, the block template would not be parsed.
    
  }

  return $template;
}

add_filter( 'template_include', 'rs_replace_create_post_block_template' );