... noticed "ATSC3-DRM SD20230420" on the System Menu tab.
.. this means the HDHR can tune to DRM channels ...

In the broad sense yes. HDHR got "certified" years ago. It was only a step in the process to get "approved" to be allowed to tune-in DRM channels. It never got approved DRM decryption ...

The problem is not just Silicondust, but all external TV turners like Silicondust. TVs work fine because they have the hardware decoder.

From what I've read, there's an issue with being able to connect/talk to the servers with the DRM keys for devices like this. It's all on the the company that ...

... I use the newest 128Gb AppleTV device on my main TV ... ATSC 3 is now in my area ... but I have no idea if it will work or not with the Flex 4k ....
... is NextGen Certified but with no DRM support ... I would even get my ATSC 3 channels ...

Yes it will pickup ATSC3 channels. Whether or ...