How I disagree about the Carte de Sejour being unnecessary.
When we moved to France, we had to have them, and in fact always had them. In spite of Prefectures from a certain point being reluctant to issue them, IF we had stayed, I would have insisted on getting them, because they are very very useful, the way the french have always been ID obsessed.
Bit late now for people to start that ball rolling, but once all is sorted, I would imagine that everyone, well UK folks in France would have to have them.
And if I disagree about the Carte de Sejours, I do agree about other post brexit paperwork, I also would say wait and see what happens before going forward, otherwise it could be endless.