We are veteran France-holidayers (my parents live there so we go at least once a year and have often tacked on a week's holiday). We're also a family of 5.
Brittany Ferries (any of the Plymouth routes) are great! The ferry really feels like part of the holiday. The cabins don't work brilliantly for a family of five though - when the kids were very little two could top and tail; then we went through a stage of taking on a rollmat, sleeping bag etc and now they are late teens we pay for two cabins (expensive, but we do see it as part of the holiday). The food is really good and we've quite often seen dolphins from the boat.
If you need to break up the journey, lots of the French hotel chains have rooms for 3 people, so you only need two rooms for the five of you - we usually go for premiere classe (which are fine), but we've stayed in formule 1s (cheaper), kyriads, and B&Bs as well.
We don't stop for lunch at service stations - we go to supermarket cafes just off the motorway (particularly like Flunches, but the Leclerc cafes are also good). Or taking a picnic and stopping in one of the aires which doesn't have services other than a loo works well too, especially with little kids who might benefit from a run around!).
We've done both Eurocamp/Siblu etc and gites - both good, depends on what you're like and what you like to do really. You can have a great holiday in either of them.
A mention for French supermarkets - very much part of the fun of the holiday; they are brilliant! If you do go to a Eurocamp or similar, make sure you go to an actual supermarket rather than the campsite one to stock up.
We've traveled on several Black Saturdays, and always found it fine. My (French) mother is always muttering darkly about it, but in our experience it's meant the roads are about UK motorway busy, rather than the virtually-empty state they're in a lot of the time. I'd much rather drive in France on Black Saturday than down the M5 on any August Saturday - it's probably nice to avoid if you can, but definitely not something to worry about if that's when your travel dates are.
Sorry, this has ended up an essay! Happy travels...