I also use Windows Server 2022 on a Futro s720 or s920, I get down to 3W with the s720. Linux is more like 5-6W. I did not have much success to use "powertop" or something to reduce power consumption on Linux, I was shaving half a Watt maybe, but nothing substantial to get anywhere near the efficient Windows idle power usage.
I guess for both operating systems, it is important to NOT connect a display or make sure, the GPU switches off the output completely when not used (not just screen blank). With output enabled (so you see something), power consumption is around 2W more, each USB device for mouse/keyboard or thumb drive, will consume 1W+ as well.
I also assume, that the very bright white power LED consumes quite a bit of power as well. I have not tested yet, how the power consumption goes down if the LED was desoldered, but worth a try I guess. o)
Sorry, can't say something about specific Linux / GPU driver, if this really is the cause for the higher power consumption when using Linux, it's worth investigating. Also Kernel 6.13 came out recently with 30% power saving potential for specific network load scenarios, not sure it also helps on an idle system, but worth a try as well I assume.