The difference here is that these are build dependencies, not runtime dependencies for the Calamares package. If Calamares needs these QML packages regardless of the slideshow format, we should absolutely include them.
As for the correct solution, I'm unclear. My gut reaction would be to add those QML dependencies as runtime dependencies of Calamares and call it a day, but highvoltage (who you should ping) may have a more well-thought-out solution that we can also upstream to Debian.
At the end of the day, we can try a couple of solutions and see what works, but coordination with Debian on this would be ideal.