I would like advice on the most SCENIC route from Carcassonne to Arles.
Should we head east on D610, D6113 or A61 toward Beziers?
Drive up to Nimes and then down to Arles?
Or take D62 and D58 across to Arles?
The A61 will not be really scenic but you will go faster (unless it's a weekend in July or August when there are a lot of traffic jams in this area).
The D6113 is neither scenic nor fast
If you are not in a hurry you can go through the Haut Languedoc Regional Parc, which will add at least 90 minutes to your journey. (in fact maybe more because you will want to stop about every 10 mi).
From Carcassonne take D620, go to Caunes-Minervois then Lespinassière.
Then Bédarieux and Clermont-l'Hérault via D920 - D908.
By taking this road you will have seen:
Lespinassière https://goo.gl/maps/QLpph542cbWMnFDZ8
Saint-Pons-de-Thomières: https://goo.gl/maps/FY4YQvXs3nAARPVK8
Olargues which is classified as one of the most beautiful villages in France and where you can stop for lunch: https://goo.gl/maps/PrHjhMrGo3ACSyeK9
Clermont-l'Hérault: https://goo.gl/maps/6TmVvD6yDSn6S9EZ9
Don't worry, the road is winding in places but quite safe. I would even say that it is safer than the highway.
Clermont-l'Hérault is 20 minutes from Montpellier by expressway or motorway.
If once in Montpellier you still have time, you can go by the seaside: Palavas-les-Flots then Aigues-Mortes instead of taking the A9 motorway. It will take you about another 30 minutes more