Well. As of today, I found the/a solution myself.
The add_action() function has a priority parameter that influences the order in which actions are processed. Knowing that the parent theme also calls this function, but w/o a priority, we can use it to ensure that the child theme stuff is loaded way after the parent. Like so:
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles', PHP_INT_MAX );
Yes it’s a hack. But it works.
