“Montmartre, Le Marais , and St. Germain des pres are central and are not too crowded with tourists.“
These are some of the most touristy areas of Paris and Montmartre is definitely not central.
Notre Dame is the center of Paris, so the closer you are to Notre Dame, the more central you are. There, and along the river, are the most beautiful areas of Paris, imo.
My favorite areas are Ile St Louis (the little island next to the bigger island where ND is), the 5th (Latin Quater), the 4th (Marais) and the 6th (St Germain).
In these areas, you’ll be able to find plenty of areas that aren’t crowded but the main streets and side streets off main streets will have lots of people and lots of tourists at the end of June.
To really get away from tourists you need to stay in the 10th - 17th and the 19th - 20th arrondissements... but they are not central.
Yes, you can take the métro or bus to get anywhere in Paris but i prefer to stay in a central part of Paris and walk to most places.