"I don’t know if this is a problem that is specific to OM System cameras, and I’m not particularly interested in investing in a new camera system just to find out."
A better way to have further narrowed down the problem down to Actually iPhoto would've been to do the same tests with a USB-C card reader plugged directly into the Mac, which would've eliminated cables, hubs, and camera hardware/software/firmware as possibilities.
It's worth noting that searches show that OM-1 USB support is imperfect, the camera manual addresses that "USB transfers aren’t guaranteed in some setups", and user consensus seems to be to use a card reader for reliable file transfers.
A better way to have further narrowed down the problem down to Actually iPhoto would've been to do the same tests with a USB-C card reader plugged directly into the Mac, which would've eliminated cables, hubs, and camera hardware/software/firmware as possibilities.
It's worth noting that searches show that OM-1 USB support is imperfect, the camera manual addresses that "USB transfers aren’t guaranteed in some setups", and user consensus seems to be to use a card reader for reliable file transfers.