Not exactly OPNsense related but since a lot of people run PiHole and OPNsense, I thought this YouTube tutorial was just as relevant as an NTP Time Source within your own local network. It can get your NTP delay measured in nanoseconds as opposed to the milliseconds that most NTP Sources provide.
In my attached pic you can see how low the delay is compared to the pool.ntp.org and NIST Servers provide.
It uses a Raspberry Pi and a GPS module with PPS signal to get a very accurate time and synchronize clocks over your network.
https://youtu.be/7aTZ66ZL6Dk?si=hhHNEFYC4jmrvbKy