Most individuals would just say, "Thank you for your patience," or "I'm sorry for keeping you waiting." Thank you so much. Or anything along those lines. To my mind, the 'patiently' element of your proposed language is arrogant. It assumes that we are all patient and not to blame if we are not. The only other option is to give them something free as thanks. Maybe a gift certificate for their next visit or something similar. You can't really go wrong with either option. The choice is up to you what message you want to send out there. But I hope this answer helps.