I’m traveling in early June. Should I reserve and buy ahead the train ticket between Aix and Dijon?
Yes if you want the cheapest Exchangeable Flexible or Non-exchangeable ticket price
Which Aix? There are several.
Aix-en-Provence
The best fares available now at www.oui.sncf are 47 euros in 2nd class, versus around 86 euros on short notice. That cheap rate also is bookable with PayPal (some other fares are not), which is an easy way to avoid the approval issues this site has with many US credit cards.