If I understand your question, you should use this inside your foreach:
$argument_to_pass_to_switch_theme = $theme->get_stylesheet();
Just taking a look at the documentation:
The function wp_get_themes()…
“…returns an array of WP_Theme objects based on the argument.”
This class WP_Theme has a method get_stylesheet(), which…
“…returns a string with the directory name of the theme’s “stylesheet” files, inside the theme root. If this is a child theme it is the directory from the child theme. “
And the function switch_theme()…
“…accepts one argument: $stylesheet of the theme. ($stylesheet is the name of your folder slug.”