Linear vs ATX

[agent_kith]

Do you think that a Thin Mini ITX board with integrated power would or could be a way of moving away from ATX power supply, or is that still switching, I could then power it using a linear supply.

Havn't done enough experimentation to know for search. At the end of the day, there's no way to get around switching as the motherboard itself probably already has SMPS in it.

What seems to be more important may well be the ground loop issue. See if you have that problem first, once confirmed you don't, then experiment with 5V power to your USB card.