Hi,
I have an old fashioned tray: there is a reservoir for filling with hot or chilled water. I use it for serving vegetables like asparagus, which stay hot as the moment they left the stove.
If your food leaves the trays as fast as I suspect, you may be spending extra money you could put elsewhere.
You can also set up some ice and leave the cold food trays on there. Or pick up at a resale shop those electric sidetable warming pads that keep food warm. They look like a formica cutting board wired with heat to keep food warm. I just don't know what they are called.
Will your warm food taste ok if warmish to room temperature? If yes, then concentrate on keep the cold stuff cold.
Regards,