Hi Leslie! While I love St. Émilion (and live close to there), I think with only 1.5 days you should spend them in Bordeaux itself. There is so much to do in the city and you can even take the tram to visit wineries in town (Château Pape Clément is one such winery) should you want to add a winery to your list. Sunday afternoons and Mondays can be a bit quieter with some shops/restaurants being closed and May is the month of holidays in France so even more could be closed depending on your dates (May 1, 8, 20, 29). Since you arrive on Sunday, definitely stop by the Marché des Quais, which is the big Bordeaux Sunday market. I think it goes until 1pm. Additionally, if you are in town early in May, stop by one of the largest Antique markets in France: Bordeaux Quinconces which runs Friday 25 April to Sunday 11 May 2025.
If you choose to visit St. Émilion and Monday would be your day, just know that not everything will be opened but there should still be plenty to fill your time as well as wineries to visit. Book in advance anything you want to do. And if you visit the Monolithic Church (it's incredible!) make sure you book the correct tour for your language needs (there are only 2 offered in English--11am and 2pm.)
Have a great vacation and hope you love it in Bordeaux!