I'm travelling from Arles to Florence in March, and I'm wondering what is the least troublesome way of doing it. I'd prefer an option in which I don't have to drive. And if travelling by train, I'd prefer a journey without too many changes, as I want to reduce the risk of missing a change.
The route with the least changes seems to be to get the train from Marseille to Milan. And get a direct train from Arls to Marseille, and then from Milan to Florence. There are multiple trains day on these routes, leave as much time as you feel you need. But the section between Marseille and Milan only runs a few times a week during the week (but daily in summer) You can check the times for it at https://www.thello.com/en/marseille-milan/timetable/
Also note, of your planning on returning the same way. The train from Milan to Marseille arrives too late for last train from Marseille to Arles. There are earlier trains from Milan to Nice and then from Nice to Marseille (adding an extra change), you could also spend the night in Marseille.