Nice Ville to Aix en Provence train ???

I'd have no problem with that transfer time.

20 minutes is enough.

Nothing to worry about as the Marseilles to Aix en Provence leg has 2 TER (regional trains) an hour up to 21:53 (9:53pm), that you can take if you happen to miss the 20 minute connection.

The best option, in my opinion, is to take the direct TGV from Nice to Aix en Provence TGV, and take the frequent bus (every 15 min) from there to downtown Aix. More convenient than changing trains in Marseille, and possibly faster if you take one of the trains that skip Marseille altogether.

Balso, thanks, but I don't want to pay what is probably a high price to get a taxi to the airport tgv station early in the morning. I'm not as worried about the taxi to town from the tgv station at Aix en Provence. Just seems simpler to walk from my Nice b & b or take a short taxi ride to the Centre Ville Nice train station mid morning, transfer in Marseilles and walk or take a short taxi ride to my hotel in Aix en Provence. Why do you think TGV to TGV station is better when they are both some distance from town? I must be missing something,

You are indeed missing something 😉: the TGV in Nice leaves from Nice Ville, like all the other trains. There is no airport TGV station. In fact, TGV and non-TGV trains use the same tracks all the way to Marseille.
And in Aix TGV, you absolutely do not need to take a taxi (the bus is really very convenient), but if you do it's like 35€ if I remember correctly.

If you do choose to go to Marseille Saint Charles, I do not particularly like the local train line between Marseille and Aix: it is slow and not that reliable. Many locals take the bus instead, which leaves every 5-10 minutes from a stop right behind the last track (I think track M but there are plenty of signs) and is faster than the train.

Thanks everyone. I booked my ticket yesterday and am looking forward to my trip in September.