How do I add something to the “Edit Tag” page in wp-admin?

There is the in_admin_footer hook that’s right before the “Thank you for creating with WordPress.” text in the footer, you can check if you’re on a tag edit screen in that hook.

function wpse_277416_admin_footer() {
    $screen = get_current_screen();

    if ( $screen->id  === 'edit-post_tag' ) {
        echo 'Hello world!';
    }
}
add_action( 'in_admin_footer', 'wpse_277416_admin_footer' );

That’s not quite what you want though, but there’s no hooks between the submit button and the footer text.