Seems like the camera isn’t the issue then.
Just realised you haven’t mentioned the results from bypassing the fathom-Xs and the tether with a direct ethernet connection between your raspberry pi and top computer. Would you be able to try that please?
If it works correctly then you know the problem is isolated to the tether itself, one of the fathom-X boards, or the connections between them, which at least rules out any issues with your top computer and the companion computer. Similarly, if that doesn’t work then you know the issue is either with the RPi companion computer or your top computer, or possibly a software issue.
You can try downloading the latest release of QGC, which is known to fix some video issues, but my expectation is it’s unlikely to solve your issue in this case as QGC in the top computer doesn’t control how the companion computer sends its video streams.
There’s a minor possibility that setting the “power line frequency” in the Camera page of the web interface to 60Hz could help, if you’re living somewhere that uses 60Hz mains power.
It may also be worth checking the network speeds when you’re using lower frame resolutions and rates, although if I remember correctly you said previously that that didn’t help (but that may have been to a different problem).