Nice build, but is that a typo or are you actually using an RTX 3090 with an SF600? 600W is almost certainly not enough for an RTX 3090, in fact it's more or less the minimum for an RTX 3080...
I actually use 3090 with SF600 without any problems. I see around 500 watts peak in games.
Peak reported power is one thing, transient power spikes are another. Sounds like you're lucky with your specific GPU, how it interacts with your motherboard, and how the PSU handles transient power spikes - but generally, 600W is not enough for a 3090 as most 3090 builds will see transient spikes well above 600W for the GPU alone. If reported GPU power draw was all there was, 600W would indeed be enough, but that doesn't account for transients. These only last a milisecond or two (and are far too fast for any software monitoring to catch), so a good PSU should be able to handle them, but that depends on a lot of specific details, especially the output capacitance of the PSU's 12V lines - and SFF PSUs tend to have fewer, smaller caps, leading to less reserve power to compensate for spikes. YMMV.
Other than that though, this is a great looking build! Really clean execution, and clearly a well matched component selection.
