Hello, There are two diners (Metro Diner and Beach Diner) close to our hotel: the Hilton Inn Ponte Vedra Beach Hotel, PGA Tour. However, our family would like to meet for breakfast on Easter Sunday, equidistant between the Mayo Clinic area and Ponte Vedra Beach, where our hotel is located. I have not called either diner about their Easter Sunday hours. I suspect the Metro Diner doesn't take reservations.

Any suggestions?

Does it have to be a "diner"?

Not in the traditional sense of a diner.

Lots of brunch options in St Augustine.

Sorry. I missed the "breakfast" part. I'm no help to you.
But I found the place that I loved in the area - Palm Valley Fish Camp
https://palmvalleyfishcamp.com/contact/
It's been a few years and I didn't remember exactly where I was, so I searched my photos.

Clark's fish camp also.

Grinders American Diner seems like a place to meet...

https://grindersamdiner.com/