This seems like the perfect use for something like a mini LIDAR you can mount on a bike that detects a car behind you and just lights up a couple of LEDs on the handlebars.
Forget the bike, I've always wanted a kit that would detect cars around my car and display them like a mini map. Maybe with high speed delta from my own highlighted.
I mostly bike around cities, and when I drive I have taken to cracking open a window because not being able to hear cars around me feels deeply disconcerting.
Every cyclist has their own preferences, but for me, a rear view mirror really helps. I still turn and look if I have to change lanes or something like that, but keeping an "inventory" of what's behind me really helps avoid unnecessary surprises.
