The
AORUS Gen4 AIC card goes for
$150 at NewEgg, and a 4TB NVMe disk would
part you with ~$800.
Initial Phison E18 SSDs seem to be limited to 2TB though, and Samsung 980 Pro only goes up to 1TB from the leaked specs.
56 Gbyte/s is quite possible with dual-channel DDR4-3600 (PC4-28800), and PCIe 4.0 x16 goes up to 32 GByte/s (in each direction).
But I don't think upcoming DirectStorage games could make use of simultaneous reads/writes on the scale of 30 Gbyte/s, because any GPU will be unable to keep up with decompression at this data rate. Not until the year 2028 - and I would rather spend on a trip to the Los Angeles Olympics than a $1500 NVMe RAID, a $2000 HEDT platform, and a $2500 Titan video card).
It would make no difference if the PCIe Switch was located directly on the add-on card, and not in the CPU Root Complex. PCIe is a point-to-point protocol, unlike conventional PCI.