@sns you will be surprised to know that the performance of motherboard usb really depends on a lot of variables, like the motherboard, chipset used, quality of onboard clock, Operating system used etc and off-coarse the big demon power supply. I have gone through a varied different types of x86 motherboard over the yrs and I am amazed to see how much different they can sound. Even which usb port you are using changes the sound. Its not just possible to comprehend and hence an audio grade quality USB pcie card solves this part of the puzzle.
On the OS, I was an avid Euphony v3 user and Windows server 2012R2 prior to it. When I changed by motherboard later at some point and incorporated a newer Intel proc, Euphony v3 sounded really bad. The v3 was using older kernel which just didn’t optimize the system in a way it should have been with newer generation of hardware. I moved back to Windows and experimented with 10 ltsc, Server 2019 and WinPE with variety of optimization over the months, until recently when Euphony v4 was released. I did the trial and it indeed sounded very good and a lot lot better than the v3, almost same as my optimized Windows. Unfortunately, Euphony v4 moved to a subscription model which is a recurring cost and not cheap either. I am still undecided at this point.
If you want to put all hands on deck with DIY, I would suggest experiment with JCAT or Pink Faun w/ ultra ocxo clock. You never know what you are missing (I used to own the Rendu). Whatever route you go, DIY or OR, getting a quality power supply (with low impedance) will be the key to how your DAC might sound.