Thunderbolt is limited to about 2750 MB/s.
If you only need this for backup then that's more than enough. Even for non-backup that's plenty for most people.
PCIe is how devices are connected inside the Mac Pro (GPU, Thunderbolt, NVMe drives, SATA and USB controllers, etc.). The PCIe slots allow adding PCIe devices. External PCIe devices can be connected to a PCIe host card that inserts into a PCIe slot. The external PCIe enclosure has a PCIe target card. Between the host and the target is one or more external PCIe cables.
The Mac Pro has PCIe 3.0 slots that range from 4 lanes (~3500 MB/s) to 16 lanes (~13000 MB/s). PCIe host cards also range between x4 and x16. Here's an example of an external PCIe enclosure with a PCIe 3.0 x16 upstream link and four PCIE 3.0 x8 slots (made for GPUs but you can put any PCIe devices inside):
Buy Dynapower USA Netstor NA255A 4-Slot External Performance Desktop PCIe Expansion Chassis with 1200W Power Supply featuring PCIe 3.0 x16 Host Interface, Single 1200W Power Supply, 8 Gb/s per Lane, Transfer Rates up to 128 Gb/s, Supports up to 4 Dual-Slot GPU Cards, Fits GPU Cards up to...
www.bhphotovideo.com
Here's some other devices (some are made for external storage, some are made for GPUs, some are made for other PCIe devices):
Your source for Expansion Chassis from top brands like Magma, ONE STOP SYSTEMS, JMR Electronics and Dynapower USA, all at unbelievable prices.
www.bhphotovideo.com
Read about this stuff in wikipedia.org (PCIe, NVMe, SATA, USB, Thunderbolt,... )
Visit the websites of the different manufacturers to see more example products (One Stop Systems, Netstor, Magma, Cubix, etc.)
External PCIe enclosures with multiple slots have a PCIe switch. A PCIe switch has an upstream connection (example: PCIe 3.0 x16) and one or more downstream connections (example: four PCIe 3.0 x8 slots). A switch not only allows adding more slots, it also converts between different PCIe link rates and widths (lanes). For example, An NVMe device is normally PCIe 3.0 x4. If I put one of those in a MacPro3,1 PCIe 2.0 x16 slot, then it can only connect at PCIe 2.0 link rate (halves the max performance). A PCIe 3.0 switch between the PCIe 2.0 slot and the PCIe 3.0 NVMe device can transmit data between the fast and narrow link of the NVMe device and the slow and wide link of the PCIe slot with very little loss in performance. There exists PCIe 4.0 back planes. I suppose you could add PCIe 4.0 slots to the MacPro7,1 or MAcPro3,1 using one of those. PCIe switches are expensive (depends on link rate and number of lanes).
Multiple NVMe devices can be connected to a PCIe slot using a card that has a PCIe switch (see products from Highpoint, Amfeltec, Sonnet, OWC...)
This one can connect 6 NVMe devices:
https://www.amfeltec.com/pci-express-gen-3-carrier-board-for-6-m2-or-ngsff-nf1-pcie-ssd-modules/
I think this one can connect 8 NVMe devices and uses PCIe 4.0 so newer PCIe 4.0 NVMe devices can be used at full speed (~5000 MB/s each):
https://www.liqid.com/products/composable-storage/element-lqd4500-pcie-aic-ssd
A MacPro7,1 has four x16 links to the CPU (slot 1, slot 3, and two of the other x16 slots divided between Pool A and Pool B) so maybe you could get 40000 MB/s (you would need a normal GPU in a fifth slot). Such an arrangement would be ridiculous though.
