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Awesome concept, but sadly limited due to a Raspberry Pi CM4 being chosen for the brains. In addition to the availability concern, there are better SBCs out there for time/GPS. The BeagleBone would have been ideal because it supports high-precision PPS out of the box.

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In this case, the PCIe card can be used completely independent of the Pi—the GPS PPS signaling can be routed differently with some dip switches on the sandwich board, and a CM4 is not necessary for full GPS + OXCO functionality.

The nice thing about installing a CM4 is it supports PTP hardware timestamping out of the box, so it could be a solution for PTP over a network if you don't want to shell out for more expensive NICs or need to support hundreds to thousands of systems.

The classic problem with PTP is that you need all your network devices to also explicitly support hardware timestamping, or you've just lost the biggest reason to use PTP instead of NTP. That may not be hard to do if you're just running your homelab network, but that gets harder and harder to do as you scale up in size.

Switches are often the hardest to find supporting PTP, it seems.

But it's gotten a little better over the years. Certainly having used hardware available now which doesn't cost an arm and a leg helps (at least in the homelab, as you say).

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