Adding my RPi NTP server was simply a matter of adding a server line to my workhorse's /etc/ntp.conf. After doing that running ntpq -p on it gave this:

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+61-68-38-238.st 203.59.7.248     3 u   10   64  377   60.369   -0.377   1.405
+103.38.121.36 ( 202.6.131.118    2 u    -   64  377   20.083   -0.977   1.474
-y.ns.gin.ntt.ne 249.224.99.213   2 u    6   64  377   14.956   +0.761   1.208
*rpi1.local      .GPS.            1 u   21   64  377    0.708   -2.365   0.296

So I will let it run for a while. Hopefully I don't get woken up at the wrong hour in the morning by my computer's alarm 😁, like the programmer who was pranked by friends who changed his computer's clock's timezone or something like that. Actually NTP will throw out outliers so if the GNSS time is wrong it will be disregarded.