You might take a look at the Garmin Varia radar. My son has been using one for 3 or so years and it works well. He gets alerted to cars well behind him. Likely further than most with mirrors. Though I don't know since I've never used mirrors for cycling. It may or may not show you other cyclists. If they approach slowly it probably won't alert on them. However if they are closing fast on you, then you'll probably get an alert.
It works well on open roads. In city traffic if probably will be more of a bother.