Okay I was able to track down the thread I was thinking of, thanks to
@MooMooVT for all this great info!
Ugh, and just as I added this to my list to visit for the Duomo stamp based on MooMooVT's experiences above, their stamps were from earlier than 10/2024, so now I don't know what to think. The Galleria Vittorio store had told MooMooVT that they didn't have it when they went, but I see in the photos on their google page they have the heat stamp machine out on the retail floor so you can watch your stamp be placed - that is how the one on Michigan Ave in Chicago is and where the unique Chicago stamp is....maybe they move it around? I just know my partner is willing to go to a store in each location with me for a stamp, but he'll be mad if we're wasting precious vacation time running around to multiple stores. I will also probably have some time to kill in FCO in Rome on our way back home, so I will probably try and seek out that stamp as well.
I still can't find any info on if any of the locations in Florence have a unique stamp?