I need to get the control choices to sanitize_callback

You don’t need the input_attrs, you need choices. This is what I do:

function mytheme_sanitize_choice( $input, $setting ) {
    $control = $setting->manager->get_control( $setting->id );
    $valid = $control->choices;
    return array_key_exists( $input, $valid ) ? $input : $setting->default;
}