Cassis (town)

To answer your logistical questions: a cab from Cassis to the airport should cost 100-120€ (Uber price being 85€ as I type this).
Also, if you have trouble with hills and with walking more than 1/2 mile, you would definitely need to stay as close to the port of Cassis as possible - it is a very hilly place - and to arrange transportation to Cassis, because the train stops 1.5 miles out of town (the bus from Marseille gets closer, but you'd have to take the metro or a cab to get to the bus stop from the train station, which is cumbersome).

I don't think that the town will be much of a change vs. what you can see the Vaucluse, besides the port and surrounding coastal scenery, that is. Marseille would provide more of a contrast, with easier logistics, and the same ability to take a boat into the Calanques ("fjords") and enjoy the scenery.
Exploring the town of Cassis itself won't take long, half a day at most, so if you have 2 full days (i.e., 3 nights), I think you might get bored of Cassis.

Don't get me wrong: Cassis is a lovely place. But I'm not sure it is the right place with the info you are giving us.

Thank you! I thought the smaller scale of Cassis might be a better fit for us, after a week of traveling all over Provence (and two days in Lisbon!), but we'll certainly think about it. If we do switch our plan to Marseille, where do you suggest we stay, given my info above.

If you choose Marseille, jt is convenient to stay close to the "Vieux Port". Tried-and-tested options include Escale Oceania and (higher end) Radisson Blu. New Hôtels Vieux Port would be good too!

And yes, Marseille will be 10x more hectic than Cassis. If you don't mind just relaxing in a very picturesque place, even without tons to do, then Cassis is a good choice! But as I said, there isn't a lot to do if you exclude hiking and water sports.

Because the town is in a bowl facing the water, with hills on three sides, everything you would want is near the water. There's nowhere for a cab to take you, except into the vineyards or up to the train station. It would be a slowed down visit with beautiful views of Cap Canaille and a ride out to the Calanques.

Any restaurant along the port in Cassis will serve fish soup. I don't remember if any serve a real bouillabaisse. The real bouillabaisse is served at a couple of restaurants in Marseille.
These are listed on the Michelin website.

Aix would be a good choice: fairly flat, a lot to visit, more restaurants open on a Sunday night.

Cassis is tiny and, unless you stay in the center, it is hilly as Balso said so you might find walking a bit difficult with your limitations. You can do a calanques tour from the docks but then what? We had a car and drove the Routes de Cretes which has spectacular views. But you won’t have a car. Other than that, there is really not much else other than a beach and some restaurants there. We stayed at the Radisson Blu in Marseilles. Marseilles used to be on the RS eastern France tour but has since been dropped. It was not my favorite place. Given my druthers, I would choose Aix.