Congrats about the steady workload, Erik. It's a good milestone.
I would say that your priority should be in getting room treatment and monitoring sorted, above all. But you mix a lot on headphones, don't you? So it may be a moot point unless you're willing to change over.
As far as the computer system goes, it seems extremely expensive to me, hardware wise. Even here in Oz you can get a PC for half that with hardware that still smokes it. Though I suppose that's the price of admission to turtleneckville.
The Duet is probably fine for your workflow. Alternatively you can look at something a bit more purpose-built for mixing like the Benchmark DAC-1 to feed your monitors off.