There are several XMP tags that DigiKam writes for ratings, but the most important one will be:
xmp:Rating="0" // values 0 (none) to 5
digiKam:ColorLabel="0" // values 0 (none) to 9.
// xmp:Label is used only if it is a Lightroom-compatible color
Rank=0 // values 0 (none) to 5
ColorLabel=0 // RawTherapee uses only ...
exiftool is probably the best multi-purpose tool for working with and manipulating image metadata.
You can remove undesired tags from image files using,
exiftool -overwrite_original -P -ImageDescription= <file.jpg>
If modifying or deleting tags in an image file, exiftool normally writes the resulting file to a temporary output file, so as not to ...
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
Most image portfolio services (eg Adobe Portfolio, many others) do support an arbitrary number of galleries, each with their own URL and protected with their own password. While slightly inconvenient, setting up one small gallery per client would work.
Also, consider simply buying USB sticks or SD cards in cheap bulk, giving everyone their files on their ...