How to pass multiple parameter in add_action()

If you need to call enqueue_assets at admin_init, then add a new add_action():

add_action( 'admin_init', [$this, 'settings_page_registration'] );
add_action( 'admin_init', [$this, 'enqueue_assets'] );

• https://developer.wordpress.org/reference/functions/add_action/

That said, if your assets are JS scripts or CSS, you should consider wp_enqueue_script() & wp_enqueue_scripts

• https://developer.wordpress.org/reference/functions/wp_enqueue_script
• https://developer.wordpress.org/reference/hooks/wp_enqueue_scripts/