Nimes as a Home Base for Provence?

I attended the jazz festival there at the arena in Nimes and it is indeed a great venue for live music.
Check out the recommended lodging in the RS guidebook -- one near the train station is also good if you have a taste for microbrew beer, 'cos that's the owner's sideline.

I stayed in Nimes for 5 nights in May 2019 at an Airbnb - it was a room in a beautiful old house, with a pool (which would be great in July). I took day trips to Arles, Pont du Gard, and Avignon. I think Nimes works well as a base and you can visit several other towns using train or bus.

Hi Jimmy, you can take a bus from the Nimes – Gare Routiere station to Vers-Pont-du-Gard – Rond Point Pont du Gard and walk (30-minutes) to see the fabulous and very famous aqueduct known simple as Pont du Gard.
You can also take a 45-minute bus from Nimes – Gare Routiere to Uzès – Esplanade and a 30-minute train from Nimes Pont du Gard to Avignon Centre. You can also take a 30-minutes train to Arles.

I'd train from BCN to Nimes but still pick up a car for at least part of the time. You are hitting Provence at peak lavender season and you'll want to explore the countryside in the Luberon and Valensole plateau. The bus/train does work for many locations, but not for that and it is more efficient to hit multiple sites in a day. Plus, at peak times it gives you some freedom to work around the peak season crowds.

The Arena is at the edge of the old town and central square and I personally would prefer that (the Nimes Centre station is not too far away -- there are now two gares). What a great venue for a concert. Try to get an air conditioned accommodation. Nimes can be among the hottest locations in July in France.

Return the car to the French side of the Geneve aeroport to avoid a cross-country drop-off fee, if you choose to keep it. Lyon is slightly more challenging with a car -- it's a park and leave it situation in the central city. But Chamonix has many neighboring side trips that are worthwhile for an extended stay beyond what is reachable by tram, lift or train.

Nîmes is in Languedoc, not Provence. However, Languedoc has a number of worthy locations to visit, i.e. Uzès, Aigues Mortes, la Camargue, le Pont du Gard, maybe Montpellier, all available by bus or train.

Avignon, Arles, and the Luberon are not far and could easily be reached for day trips but I recommend a car rental for Provence.

Nîmes is in Languedoc, not Provence.

Officially Languedoc is no more; it is as of 2016 called Occitanie. But Nimes is a good base to explore Provence.

Officially Languedoc is no more

Let´s not confuse rather large governmental administrative regions with the nomenclature of an area's cultural heritage. Using your logic, Provence is no more.

Many would agree that Provence itself is an even better location from which to explore Provence.

My husband and I visited Nimes in 2013 as a day trip and liked it very much! We loved our self guided tour of the roman colosseum there; the signage in English was excellent. Even if you've been to Rome's colosseum, this did not disappoint. On our way back to where we were staying, in a home exchange home in a lovely non-touristed village called Pelissanne, we stopped at the Pont du Gard. If you really like to experience a place fully, I recommend bringing/wearing a bathing suit so you can swim in the exhilarating cool river that runs beneath it. It will be hot in July, so it's super refreshing! That's my two cents!

We have family in Uzes, so we've done some travel in the area. We like Nimes very much, and there is bus service, but we found it easier to rent a car. We rented at Sixt which is right behind the train station. Provence is a bit far for basing in Nimes unless you have a car. We stayed in Uzes, which is worth a visit especially on market days. Don't miss Pont du Gard. We also like our visit to the Camargue. But something about the south of France makes us slow down. We have days where we just sit in the square in Uzes.

We're going back next year and hoping NOT to rent a car and I'm finding getting from Chamonix to Nimes on transit is complicated. It's easier just to go to Geneva. So beware of that.

I loved the amphitheater and think it would be very fun to see a concert there. Enjoy!

Nîmes is a great base, and 3 full days will give you time to explore Arles, Point du Gard, Uzès, and Nimes itself. Only an example, there are so many places...
Whether you need a car is debatable. It provides flexibility, but if your budget is tight or driving abroad stresses you out, you can do without!