Question on how to get to Biarritz

I've not done this tour but the fast trains to Bayonne go thru Bordeaux which I found to be an interesting city.

I would vote for a few days in Paris and then down to Bayonne at least the day before your tour starts. I'm pretty wary of train strikes in France so I always allow myself a few days flex time, lol.

You can also fly directly into Bordeaux - there are flights on Air France or KLM from either Paris or Amsterdam, depending on where you are flying from in the US and which European hub makes the most sense.

Still....I'm a Paris person so for myself that is always a good idea, hahah!

I find Biarritz to be a soulless high rise kind of place, admittedly with a great beach.
Bayonne, however, is charming. A town set at the confluence of two rivers, with cobbled streets and great shops, bars and restaurants.
There is a good bus service if you are staying there for a few days and want a quick trip to Biarritz, or to Cap Breton, a nearby small resort with windsurfing.

This is what Rick Steves says on the web page about the tour under the tab “hotels and flights.”

“Most tour members arrange to fly into Biarritz, France (BIQ), and out of Bilbao, Spain (BIO) — choose "multicity" on flight-booking websites. The Biarritz airport is a 20-minute bus ride from Bayonne, where the tour begins. Booking round-trip to one city will require a connecting flight, train trip, or additional overnight stay in Europe.
We suggest arriving a day or two before your tour begins to allow ample time to get over jet lag and be well-rested for the first few days of your tour. For more information about transportation options for getting to and from your tour, see your Rick Steves Snapshot: Basque Country guide.”

When we took this tour in 2022 we flew into Paris, and then flew domestic to Biarritz.

Since we came in a 3 days early, we took a taxi from Biarritz airport to Saint Jean du luz and spent it there. Its a short taxi ride.

A wonderful place to get over jet lag and to get my high school French working again.
We stayed at the Hotel Grand Thalasso- Right on the beach.

St. Jean du Luz has a nicer old town charm to it than Biarritz.

Though English is widely spoken in the tourist sectors, our taxi driver spoke French, Spanish and Basque, but very little English.

Then we took a pre-booked taxi van to take us to Bayonne to start the tour.

I was in Bayonne recently and used it as a base and took trains and buses around. I agree with the responder above that Bayonne is far more charming than Biarritz. If you choose to fly in then the taxi ride from BIQ to Bayonne is indeed short and so not pricey. If you choose to stay in st jean de luz then you could fly in to BIQ and taxi there. or take the regional bus. it is easy to take the regional bus directly from there to Bayonne. All the local bus info is on the TxicTxac regional bus app. (TxicTxac is name of the super useable regional bus system). It is also easy to bus between Biarritz and Bayonne.

As for train strikes, I agree that it is good to be aware of the potential for train strikes. No need to panic but is a good reason to fly in. Note that (from what I hear) when a strike is planned it often falls on a weekend and includes an extended holiday weekend. For example, today is Thursday May 8 and is a French holiday and the France SNCF Train unions are on strike from about yesterday and likely through the weekend. You may want to check to see if any official French holidays fall near the time you may be arriving to see if this might be an issue for you. I very much enjoyed my time in Pays Basque and San Sebastián and I hope you will too.