Hi,
I like all of the cities you've mentioned and have been fortunate to visit them all.
If it were me, I would use Avignon as the base. There is ample there in the city to keep you busy for a full 24 hours: the Palace of the Popes, the Pont d'avignon, and the old town. In addition, you should really try to get to see the Pont du Gard which is accessible by bus.
From Avignon, you can visit Arles, Nimes, and/or Aix. Each of them are worth a full day and are all accessible by train. I liked Nimes and Arles because of the ancient Roman amphitheaters in each city, plus Nimes has the Maison Carre, the Castellum, an ancient Roman Tower (and other ruins) and two beautiful public gardens. Arles has other ruins, plus lots of Van Gogh sites and a great Museum of Roman Antiques. All three have great plazas, restaurants and wine bars.
Aix is very beautiful, too, with some very beautiful museums and Paul Cezanne's atelier, along with some great churches, plazas/squares, and great scenic views of Mt. Victoire. I'm not sure if all, if any, of the sites I mentioned are open during Covid, but all the cities are great.
Best of luck