Is their a way that Garmin can adjust the distance detection of the RTL510/511 via software update? maybe 500 meters would be sufficient.
A cyclist doing 25 kmh and a car doing 100 kmh gives a closing speed of 75 kmh, or 20.8 m/s. About 7-8 seconds warning with the stated 150 meter distance.
500 meters would mean about 25 seconds. On an even moderately busy road this would mean your screen would constantly be filled with warnings, and you would end the ride a nervous wreck just from the anticipation. As a cyclist, the greatest threat to your safety is hitting obstacles, potholes, road debris, other cyclists in a fast-moving bunch etc. and most of the time you should be looking at the road surface, not at your screen.
In my mind, Garmin have got the balance about right between providing sufficient time to take action, and focussing your attention on where it should be at any given moment.