I am not very familiar with Kotlin Multiplatform, but couldn't you use a Swift/Objective C library for iOS, while using a Kotlin library for Android? What would be the blocker to this?

There's no blocker, but that's not a comparable development experience to more mature cross-platform toolkits.

I now have a separate QR scanning implementation for three platforms and a bunch of if statements in the Gradle files. It took 4x as much effort (once for each platform, plus the work to tie it all together) and now I also need to know how to develop for those platforms, which is exactly what I wanted to avoid by picking a cross-platform toolkit.

This will hopefully improve with time, as it did with RN and Flutter, but my point is that we're simply not there yet.