although 575-BBSI isn't available so I'm trying to find the constituent parts and am slowly making progress.

     I already sent you a link to the rear flexbay kit on a different thread

However, also on that page under part 575-BBSP and 575-BBSH it says I could install a couple of M.2 drives attached to PCIe and then using 575-BBSL turn the 5.25" bay into a pair of 3.5" bays able to take more SATA drives (as long as the rear storage bays aren't used).

     Flexbay 2 was intended to use with LSI controller.  When you use flexbay 2 with integrated controller, it will take up the cable/connector from flexbay 3 and 4.

......there not being enough Motherboard connections for more SATA drives if all 4 Flexbays are fully populated as that would be 8 drives in total

     There are propriety connectors and cables to connect to each flexbay, the bandwidth and lanes designation are plenty but they got constrained and limited by these connectors/cables.  Using aftermarket adapters and splitter cables may work.

2 x M.2 connected via PCIe in say Flexbay 0 (please if someone could let me know if this is a direct MoBo connect or if it's a PCIe board I would appreciate it).

     Unlike the 5820 and 7820, the NVMe/U.2 connectors of 7920 are not on the motherboard, they are all arranged together with SAS/SATA connectors, fans, and sensors on a daughterboard located on the other side of the motherboard.  Flexbay 0 with NVMe/U.2 backplane is connected to x2 miniSAS HD connectors located on that special daughterboard.  Each connector and cable manage 4-lane direct to CPU.  (To connect all 4 PCIe SSD via flexbay 0 and 1, it requires both CPU installed.)

8 x SATA drives in say Flexbays 1, 2 & 3 + the new Fexbay that 575-BBSL generates in the 5.25" bays

     As mentioned, 8x SATA drives are designed to populate x4 in flexbay 0,1 and x4 in flexbay 3.4.  While there are 2 more SATA cables for optical drives (5.25" + slim), they can't be use to fit the conversion kit for flexbay 2.

I realise this is messy, but if I can commit all of the 8 drives fully to a MDADM array with the M.2 drives acting as boot drives that would be my preferred solution

     For your intended usage, 8 drives can be populated in flexbay 0,1,3, and 4 as each bay can accommodate 2x of 3.5" drive.  Your boot drive can be on an NVMe SSD connecting to PCIe slot using aftermarket adapter card or Dell Ultra-Speed

I do also know there's a Dual High Speed PCIe card that I can mount a couple of M.2 cards onto, but if I can mount them in a Flexbay that would make changing drives so much easier if it ever becomes necessary..

     You can mount NVMe SSD to Dell flexbay kit or you can mount it on aftermarket U.2 adapter.  PCIe NVMe flexbay backplane is required.

If you didn't want to order a system with all the needed flexbay kits offered from Dell, you must be upgrading your existing unit.  You don't mind sharing the current configuration of your system so I can offer better suggestion to meet your upgrade needs.  Below is storage options for 7920.

I think you misunderstood my comment about 575-BBSI, I meant that I'm slowly getting the components of that kit together as the kit is not currently available from Dell themselves.

I also stumbled across this document in my searching....

https://www.delltechnologies.com/asset/en-us/products/workstations/technical-support/Precision_7920_Tower_Technical_Guidebook.pdf

I thought I'd posted in one of my earlier messages that I had already purchased a used T7920 at a great price.  It is a pretty good spec with a pair of Gold 6138 CPUs, 64Gb Ram and a P4000.  It however lacks the extra storage space I want, thus the search for the Flexbay cages and trying to figure out what is needed to configure the system exactly the way I'd prefer.

I'll also be installing a 10Gbe Ethernet Card that I pulled from another system.

Currently it only has Flexbay 0 and 1 installed and after reading your comments and taking onboard the possibility of using 'extra' cabling my hope is to install 2 x NVME M.2 Drives in Flex 0, 8 x Sata drives in Flex 1-4 (after removing and replacing the 5.25" bay to convert it to Flex 2).  I'll need to track down the appropriate backplane for the Flex 0 M.2 drives so if you have a Dell part number for this that would be useful.

I don't intend to purchase a 9460-16i card as honestly my experience with MDADM under Linux has been fine and the hardware card would be an unnecessary expense, unless it's needed specifically to add the extra connectors in which case I'll have to reconsider.  I'd looked at both the High Speed Duo and Quad cards, but would much prefer Flexbay mounting drives if I can go that way.

Hope this clarifies.

For the Flexbay NVMe backplane you'll need to search for Dell part 076W3. I found it on Amazon.

For the module(s) that would go into that bay space you'll need two of Dell part 066XHV. I also bought two of those on Amazon. They come with the NVMe carriers.

I have Flexbays 0 and 1 as both NVMe drive bays (1TB in ea with vROC raid) as well as the 4 NVMe adapter card that goes right into a PCIe slot (Dell part 80G5N) also from Amazon with 4 Kingston KC3000 2TBs in each slot of that card. It all works fine.

Could you let me know, the Flexbay NVME backplane, does it plug into a SATA socket on the mobo or does it have some kind of dedicated PCIe port/connector?

If it plugs into a dedicated socket I'm not sure why it wouldn't also be possible to install 8 SATA drives, unless the PCIe/SATA paths are shared as they are on some other single CPU motherboards.

It does come with specific PCIe cabling. 

Thanks, could you let me know where on the Mobo it plugs in so I can look at the connector on my unit?

Found another useful video for installing rear flexbay.

Info was provided from original response.  Connecting 2x M.2 to 2 miniSAS header located on the back of motherboard.  There are x4 headers at the top, right next to the power supply.  Beside the data cables, there are (temp) sensors and fan cables as well.  The board has all the marking for flex, temp, sensor.

The part numbers provided by the other member is correct one.

Edited:  Found a useful video with part numbers

Yup, those are the one.  Each connector/cable has 4-lane.  Each PCIe/NVMe backplane use x2 connectors/cables. 

I do not like your setup plan as it requires to search and acquire all three kits.  I would go with my original suggestion:

• Get a rear flexbay kit, plus your original front flexbay =  x8 SATA drives
• Get a PCIe adapter for your (NVMe) boot drive.

Uhm,

if the idea is having 2 nvme units that can be swapped out without opening up the computer, there would be also this option

ToughArmor MB840M2P-B 

Add 2 of these in 2 pci-e slots ( sacrifices 2 slots, yes ), and as Chino de Oro , use the standard Flexbays for the 8 sata drives 

There's also the MB842MP-B , that allows to install 2 nvme with a single card 
https://www.storagenewsletter.com/2022/10/21/icy-dock-tougharmor-mb840m2p-b-pcie-3-0-4-0-x8-or-x16-card-for-two-removable-m-2-nvme-ssd-bays/