Hi OP!
Good progress diagnosing that.
Don't worry about your cartridge. This is telling us you have a noise issue, either radiated noise from the TV or it's an AC issue. The TV is putting noise into your AC line which you are picking up.
Phono preamps are incredibly sensitive to this. Make sure you are using fully shielded interconnects. So the next question is,m is your TV using Wifi or Ethernet? If wifi, try switching to Ethernet as an experiment.
If it's already on Ethernet and still causing you issues, you may want to use a good power conditioner, but honestly this solution is iffy. Better to relocate your gear away from your TV. If you do use a power conditioner, keep your TV outside of the cleaned outlets.