Note that for NTP it’s better to use a Raspberry Pi 4 than older boards. The old ones have their ethernet port on the wrong side of a USB hub, so their network suffers from millisecond-level packet timing jitter. You will not be able to get microsecond-level NTP accuracy.

For added fun, you can turn the Raspberry Pi into an oven compensated crystal oscillator (ocxo) by putting it in an insulated box and running a CPU burner to keep it toasty. https://blog.ntpsec.org/2017/03/21/More_Heat.html (infohazard warning: ntpsec contains traces of ESR)

That's a Stratum 0 server since it gets its time from GPS. Stratum 1 server is one that gets its time from Stratum 0 servers.