What's with the patching? It sounds like pretty much spot on for what NTP was designed for.
Each clock is monotonically increasing, with its individual drift. NTP will pretty soon find out exactly what the right polling interval is. Even if you run it in broadcast mode (why would you though, with only 70 devices).
It's been eight years, but if memory serves, the primary need was to force it to broadcast as frequently as possible.
Everything you said is totally legit unless you're addressing devices with software clocks that are running at speeds dictated by subtly shifting voltage.
Remember, according to the Pi itself, it's 100% in sync. No problem hereeeeeeee (robot voice as voltage dips to 4.9887 volts).
Each clock is monotonically increasing, with its individual drift. NTP will pretty soon find out exactly what the right polling interval is. Even if you run it in broadcast mode (why would you though, with only 70 devices).