THEY LOVED IT!
When they opened it, and read what I wrote on the back, their eyes misted up, and they came over and gave me a great big hug. They both said they were secretly hoping that I would do something Zentangle related for them as a wedding gift, but never thought it would be as big and detailed as it was.
All in all I feel really good that they are happy with their gift. It took over 60 hours of work, so I got a print made up so that I’d have a copy for my portfolio.