How to output get_tags array list to select box

You can do it like so — replace the $tags_array = get_tags(); with this:

$tags_opt = array();
foreach ( get_tags() as $term ) {
    $tags_opt[] = array(
        'label' => $term->name,
        'value' => $term->term_id,
    );
}

And then, for the “Select Box” field, set the options to $tags_opt like this:

array(
    'label' => 'Select Box',
    ...
    'options' => $tags_opt,
)

If you want to save the term/tag slug instead of its ID (term_id), change the 'value' => $term->term_id to 'value' => $term->slug.

And if you want to include tags without any posts, use get_tags( array( 'hide_empty' => 0 ) ).