"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 poss…
This. I work as a professional photographer. Nikon shooter. I only ever import directly from my camera in very rare cases. And when I have a problem importing directly from card to Lightroom, first step is to copy everything to the hard drive and import from there.
Never use the camera over USB, the experience is terrible on everything from Canon to Sony to Panasonic to Fuji.
Don't fight it, just buy that $30 USB hub and get on with your day.