I too have been watching this series and have also been intrigued by the great variety of vehicles
I can’t date the non Citroëns but the Tractions all appear to be pre-war models and I did spot one post-war 2CV AZU.
Simenon’s novels were written between 1931 and 1972. Various clues suggest Maigret was born in the 1890s and is understood to be in his late 40s in a number of the novels. That would tie in with pre- or early post-war timeliness.
I also note the police vehicles are generally Légères and not the 15/6s we associate with Rupert Davies.