It's hard to get much more accurate than Chrony with NTP. Even real-world PTP implementations don't often aim for more accuracy than is possible with Chrony.
Because it turns out NTP can be much more accurate than most people realize.
From the Chrony FAQ[1]:
> When combined with local hardware timestamping, good network switches, and even shorter polling intervals, a sub-microsecond accuracy and stability of a few tens of nanoseconds might be possible
Good network switches and NICs with hardware timestamping support are commonplace now in server environments. NTP with Chrony is pretty hard to beat in terms of simplicity, reliability, and accuracy.