If you could find an off-the-shelf ADC with those capabilities, I'd love to hear where. Last time I checked, I was unable to find anything that would financially make sense compared to just buying the entire oscilloscope, but I may not know where / how to look.

everythign covered by itar is super expensive

https://www.federalregister.gov/documents/2024/09/06/2024-19...

"A resolution of 12 bit or more, but less than 14 bit, with a “sample rate” greater than 400 MSPS;"

The EEVblog teardown of the DH800 shows that the ADC is a custom Rigol part. Whether it's a rebadged ADC or not is unclear, but I have my doubts that the price point of the Rigol can be hit with off the shelf components from the North American supply chain.