Is this not recommended? Departing Marseille on a 11:30 am flight

You are fine staying in Aix the night before your flight.

The airport is halfway between Marseille and Aix. No issue at all even for an 8AM flight, let alone an 11:30AM flight.

Marseille Provence airport is an efficient operation, including its website:

https://www.marseille-airport.com/

Click on Access to see various ways to get there from Aix-en-Provence. You certainly do not have to go through Marseille. The frequent bus service, for instance, taking about 30 minutes. Taxi or limo would be luxury but not a huge expense.
For any long international flight, two hours in advance is a rule-of-thumb. Trying to arrive at the last minute costs more in stress than any time saving is worth. Your airline website may give more authoritative advice concerning airports it serves. Anyhow, arriving early allows time for a stress-free airport breakfast.

Thanks everyone for the sage advice!

There is a shuttle bus from Aix to MRS, every 30 minutes.
https://www.marseille.aeroport.fr/parkings-et-acces/acces/bus/aix-en-provence

I'm usually at MRS two hours before my international flights (international within Europe - in or out of Schengen area), and it is usually incredibly dull (1 hour would be plenty of time almost always).

The tricky thing at MRS is finding the right terminal building (1 or 2, completely separate) and then finding the right security line in that building (in the terminal with the budget airlines, there is only one line). If you go through the wrong security line, you may have to retrace your steps... However, in my experience, there usually a lot of very helpful staff around, who will actually read your boarding pass and alert you that you've messed up, and explain to you where to go. :-)