I have no use for the smart health thingies, which really look like a data driven health gimmicks to me.
NFC on the other hand I could find hundreds of applications, from payment to access and transport cards.
So for anything other than systems you control or are good friends with the IT guy for, you're out of luck.
The things is 99.9% of access cards (where I leave at least) are default-encrypted mifare classic, making cloning trivial. Transport cards are an other beast since they have their own backlog and proper encryption, but there are ways.
So all in all, dumping the card is not the issue for me, it's the medium on which to put the clones that is still a question mark.
The "NFC sticker on the back of the phone" is cool because it's almost as if your phone opens the door (stock android won't let me easily swap NFC SC ID), but NFC is fidgety when multiple chips are in close proximity, leading to frequent misses.
I have found multi-chips NFC cards on Ali Express. These are basically a single antenna wired to an array of chips directed by a keypad. That seems viable on paper but you still get to carry the card and press the right switch.
The ideal solution would be a smart ring with a reflashable NFC chip, along with a programmable MCU to implement the rolling logic between cards.
Unfortunately, no.
From my experience at least, most access cards are simple mifare classic cards, and they have no payload: the reader just got a list of allowed card IDs, maintained by the building IT.
While you can freely rewrite mifare data from Android, it won't let you change your ID unless you root your phone. I guess this is similar to the old days where you weren't supposed to change your MAC addresses.