This is pretty easily worked around:
https://forums.macrumors.com/thread...-for-pcie-slot-2.2134562/page-2#post-26413743
Currently have Mojave 10.14.3 18D109 with Sapphire Pulse RX 580 8GB and using all available PCIe & SATA.
Slot 1: RX580
Slot 2: PX1 NVMe with EVO 970
Slot 3: Apricorn Velocity Duo X2
Slot 4: USB3
4x SATA sleds: SATA SSDs
SATA optical bay #1: DVD
SATA optical bay #2: SATA SSD (no mount)
Setup with PX1 is nearly identical to those images posted when was using Apricorn Velocity Solo X2 as boot, but the thermal pad stack placement has shifted slightly. PX1 has screws on bottom in a few locations, but otherwise is extremely smooth. No issues using it at all. No GPU fan interference or knocking.
Also as an FYI, the RX 580 GPU is dead silent the majority of the time. Fans run at machine startup and that's usually about it. Adobe CC 2018 (AE/Premiere Pro/Media Encoder) rarely "kick" the fans on when normally engaging GPU acceleration. Typically only kick on during heavy renders. (Unfortunately, less options with Metal acceleration vs. CUDA, but that's another topic.)
Overall, the machine itself is extremely quiet without spinning HDDs present. Often more silent than MBP11,3.