You did not give the month of the year, which is important because of crowding in anniversary months, and in peak tourist season. Prowling for a parking space can take some time, with a walk to the actual cliff or beach still required.
I personally did not care for the affect of the highly-skilled paid guides we heard during our independent visit to many D-Day sites. They were loaded with detailed information, binders of photos and enrichement material, and knew the answers to every question. But their intense, Sergeant-Rock delivery was not to my liking. I don't mean to penalize someone for their "passion". But you did ASK for thoughts. I will add that my former Opthalmologist couldn't say enough good things about his (similar) tour, with his family.
We used a couple of books, which worked well for us. I have no idea if it's still possible to make the rather dangerous (but no "equipment" descent of the ranger-ascended bluff, but that was a high-point for us. We are able-bodied, but not hikers or rock-climbers. There were flimsy, damaged aluminum ladders in place some years ago. The sign warning you not to continue was valid and accurate.
I guess you might want to decide on concentrating on the excellent museums, or on the vast number of sites. There's a certain difference between looking out at distant rusting landing craft, and peering directly into rusting gun emplacements.
This is certainly not a complete list, and "old" books have to be regarded with accuracy concerns:
Bolger and Stoller, Fodor's Short Escapes in France, 1966, ISBN 0-679-03071-9
Frommer's Frances Best-Loved Driving Tours, 1997, ISBN 0-02-861569-7
Lonely Planet France, current edition.