Seems like the gap between 4 and 6.8 GHz was never closed (known since initial design in 2019). It appears that the main limitation on the mirny side comes from the programmable attenuator that only goes up to 4 GHz. https://www.analog.com/media/en/technical-documentation/data-sheets/hmc542b.pdf Were there any designs that covered the full freq range but dropped the variable attenuation capability?
Related docs / discussions:
https://m-labs.hk/docs/sinara-datasheets/4456-4457.pdf
https://github.com/sinara-hw/mirny/issues/1