So excited for you! I would recommend reservations for dinner. After a busy day, it's nice to know you have a table in a nearby restaurant. Not sure what your budget is so check menus. Here are two suggestions:
La Mediterranee
https://www.la-mediterranee.com
they have indoor and outdoor dining
We saw a very famous American actor here :) no names
La Ferrandaise
http://laferrandaise.com/english/home.html
indoor dining. tables are rather close to each other, but the food including dessert is wonderful. We always go while in Paris.
Cafe Varenne (in the 7th but walkable) you'll be walking a lot :)
https://menuonline.fr/cafevarenne
we have not been able to book a spot at this restaurant but others recommend it highly. Open for breakfast thru dinner
There are a lot of cafes in St. Germain so you can check outside menus or daily specials and decide on the spot. No need for reservations.
Also in the 6th, but farther from your location are a couple favorites of ours:
Cafe St. Placide (Arr 6) indoor and outdoor; casual, good service
127 rue de Rennes
https://restaurantguru.com/Le-Saint-Placide-Paris
Villa Medici, Italian indoor and outdoor; casual, excellent salads pizza
Suggest combining with a trip to Bon Marche, just one block away
11 rue St. Placide
https://www.restaurant-villa-medici.com
While I have you, you must go to Luxembourg Gardens while in Paris.
Have a wonderful time.