Any thoughts about the ending?

I think it was closure for both of them. They both shared the same man, and in a way they are connected.

It doesn't really give us any insight on whether or not Isabelle will continue, but I think she will not relapse.

When she picks up the phone and swaps SD cards, we think she might be going back to that lifestyle. But she shows up to the same hotel in her normal clothes, not her business casual with heels look.

Then she has the scene with the wife, and I think that shuts the door on that lifestyle.

I agree with what you wrote and i believe that the ending was about closure. Also you meant "sim cards" by "sd cards".

It marks the start of her becoming a lesbian

I'm not sure, either, but I think we're not supposed to be sure about much in this fascinating film.
However, I do agree w/ first responder that closure has a lot to do w/ the Rampling scene.
I also agree w/ subsequent responder that it sort of raises the question of lesbianism, although Isabelle/Lea and Rampling character don't act on it. The film 'explores' sexuality in a sort of parallel way to Isabelle/Lea.
Rampling has bigger roles in a couple of Ozon's other films. So, for me, there was a shock of recognition seeing her in this one, briefly, at the end.

Did anyone else think the ending was just a dream? My initial thought was that Isabelle was dreaming the intereaction with George's wife.

Just closure. I thought it was a poor ending to a good film. A far better one would have been for Rampling's hand to end up around Isabelle's throat.....

Probably really because it was difficult to come up with a good ending.
He could have her regret her ways and take up with the new boyfriend for good, but that would be too trite.
He could have her life ruined by what she did, but that would have been trite as well.
So in the end he went for this unexpected and somewhat weird finish. I think the key is not in the finish, but in the body of the film. Don't give the ending too much importance.

Maybe one's interpretation is based on age. When I saw the lying in bed together and the older woman stroking the younger woman's face, I felt I understood the message...to me, anyway. It's about the older woman looking wistfully at the younger one, remembering perhaps her own "sexual awakening" and young love/sex. Eventually we all grow old, sex becomes less frequent/more difficult (old man was using viagra, etc). There is something special about being young and at the beginning of your journey into sex, discovering yourself, etc. I thought the scene was very touching (no pun intended, hahaha).