With any kind of feedback from the ground (to compensate for atmospheric effects), I think these could easily be coherent. One just needs timing accurate to 100 picoseconds within one 20 millisecond round trip - which is 5 ppb and the cheapest atomic clocks can do that.

Even so, my guess is you aren't going to be frying an egg 500 km away even with the whole starlink constellation at full power!