Depending on how much you were planning to spend….
The best USB cable I have tried is the Audience AU24SE+.
What to upgrade first? Tough to say…if you get better ICs you can only realize as much as your DAC and USB cable can dish out, and other way around if you upgrade USB your ICs may become a bottleneck.
In my opinion, best way to approach it at this point in your system is to upgrade the DAC. Forget cables for now. But again, just my opinion.