[DoctorOetker]

add on top of this that oven controlled crystal oscillators (or any more performant technologies if affordable) would be selected by militaries...

[eqvinox]

I'm unaware of any technology between OCXOs and Rb standards. The latter have gotten smaller but not tiny and also need quite a bit of ongoing maintenance and calibration.

[raron]

Microchip has some "chip-scale atomic clock"s, not much bigger than an OCXO, but a lot more expensive.

https://www.microchip.com/en-us/product/csac-sa65