(Long time French resident here)
Take your EHIC cards but ensure you also have adequate insurance to cover for eg repatriation or emergency transport in the event of an accident. (Ambulances are not necessarily free in France).
You don't really need breathalyser kits: the law was never fully implemented. You are supposed to have them but there is no legal sanction available for not having them. We don't bother, they go out of date quite fast.
You do need a yellow vest and a warning triangle for your car. It's also recommended to carry some spare bulbs, cars here always come with these.
Remember that on single carriageway roads the speed limit is now 80 kmph, not 90.
If you avoid driving in old town centres you will probably avoid coming across the priorité à droite rule (don't worry too much about it).
Not yet widely enforced but it now illegal to eat or drink at the wheel, as well as using a phone of course.
Swimming costume rules vary from place to place. We have found that unguarded campsite and hotel pools (outdoor) often tolerate beach wear. Take both sorts and also caps if you think you might swim at an indoor pool.
Carry photo ID documents everywhere.
Highly recommend bip 'n' go for péage especially if travelling at busy times (Fridays and weekends especially). But not strictly necessary.