I feel really dumb, but I just learned something: The antenna that came with my PC's motherboard is for both WiFi and Bluetooth.
I thought it was just for WiFi. So, when I've had the PC connected to wired ethernet, I disconnected the antenna.
This whole time I've had awful Bluetooth performance and I had no idea why. Sometimes it would work great, but for some reason I never associated that with the times I happened to have the "WiFi antenna" connected.
🤦♂️ 🤦♂️ 🤦♂️ 🤦♂️ 🤦♂️