Alright, so I tested the SCART cable again and was able to get a 480p signal working. However, it has the same aspect ratio problem as the VGA cable.
The Dreamcast startup screen is close to 4:3, but it’s slightly narrow. Once the GDEMU loads up, and when I start a game, the image gets much narrower.
At this point, I’m convinced my TV is simply incompatible.