You could prepare the cards with the QR in advanced.
Depending on the knowledge you have in handling web hosting services there are some options.
- A randomized link.
Let's take for example this initial code DateEvent+SequentialNumber+RandomString
1521.jpg to 2020-01-01+058-4BTISt77SCg1DlnzfQdc
- And generate a QR from it
You can identify the card looking at it, and read the photo number from your camera, and upload the photo with that name, on your server. Make sure your server does not show the directory listing
You can switch SD cards with an assistant, or send a jpg file using Wifi.
Give some expiration date, let's say one week after the event, so the files are not there for all eternity.
A bit more complex would be randomizing the name itself with some encryption method, making directories/username/password and uploading, making yourself an application that also generates the QR... but that depends on your knowledge.
Beware fancy QR generators, because normally they send you to their website first and then redirect you, making the QR dependant to them. These two (As far as I tested them) generate a direct link QR.
I have used the first one for some years and the QR is clean. The second looks nicer but I have not used it extensively, just tested it.
If you include the date on the card, they will be useless if not used.
As an alternative, you would need to relink the random card to the number of the photo itself. You could take a picture of the person holding the card you provide them.