@Hangman I tried rasterizing nothing because that's what someone earlier on this thread said was the solution to removing the gray crop lines. But as you say, it really doesn't make sense for what I'm doing, so I'm back to square 1 in terms of finding a solution for how to remove the gray lines without messing anything else up - so I'm sticking to the rasterize unsupported properties export setting.
I did figure out a way to remove the gray lines but it's a bit involved. I've outlined my steps below. I'll be using this program a lot more for similar stuff though so if there's a more straightforward way to removing the gray crop lines when exporting to pdf that would be excellent to find out.
1. In affinity photo, auto white balance for the image in question
2. Click on flood select tool. Find the right tolerance to catch all subject matter and just select the white area
3. Flood select the white area next to where the gray line appears
4. Press delete (nothing happens visually but once I export to pdf again the gray lines have disappeared).