I feel that a rider with a mirror and one ear bud is safer than a rider without a mirror and no buds.
The safest way for me to cycle on the road seems to be this;
Don’t hug the curb or far right side of the lane (America). If you hug the right side of the lane the motorist that’s only looking up occasionally may not see you. You tend to blend in with the bushes, trees, signs, etc. Hugging the lane also encougages drivers to go around without slowing IMO.
Constantly listen and check my mirror. If I see oncoming traffic I make sure I take the whole lane if there’s not room for a car to pass safely. Sometimes, if there’s a safe gutter area I will pull over and let the car pass if it’s uphill (after they’ve slowed to my speed).
My goal is to always watch the cars behind me switch to the other side of the road. This is not possible without a mirror.
I choose two lane roads over 4 lane roads. This feels safer for me in our area.