I don’t drive in France, but there is already Train stations with parking, Centre d’expositions for example?

Park and ride require considerable space so often it might be preferable to park in a station that make the remaining journey okay or take the train directly.

they might not be even biodegradable, not unless they separated which is impossible to do

Ahh a classic vehicle cyclist!

However, most tourists only experience the public transport in relatively central areas of the city, going mostly towards the centre, and mostly outside rush hours, so they do experience it at it’s best.