Half day tour to Arles from Aix ?

I'm looking at my older Provence guidebook and for Aix see Pascale Benguigui and Dominique Barte listed "good choices for this area". Maybe they could take you to Arles? Also the blurb says "also consider Catherine D'antuono and Sarah Pernet who run Discover Provence tours. https://www.discover-provence.net

I personally enjoyed Arles a great deal, less for the "Van Gogh sites" than for the city's rich history. (And FTR I am a huge fan of Van Gogh.) Make sure you visit the Roman colosseum there and get the history of its use from Roman times to today.

I think your proposal is quite feasible. You may want to get yourself to Arles yourself via train; it's about a two hour ride changing in Marseille.

Three of us took an Uber from Aix to Arles last September. It was approx. $120 one way, and a driver responded promptly. The drive took around 45 minutes. We were staying in Arles for 3 nights, so didn't need a return or a private guide, but just wanted to share it as an option. I find Arles to be a very easy city to navigate, and 6 hours gives you plenty of time to see many of the sights. If you didn't want to spend the money on a private guide for the whole day, Uber/taxi might be another option.

On Friday, March 8, I took the bus from Aix to Arles. It was about an hour and a half with the change in Salon de Provence, very easy, very quick, very nice trip. It was much easier than traversing the Marseille Saint Charles Train station. The bus station area in Salon, you just have to walk a few feet to transfer from one bus to the next.