T5820 bootable NVMe upgrade

Hi,
Running a T5820, Xeon W, with flexbay and PCIe, latest BIOS and Win 10 Pro. Stock Bootable PCI-e drive is a Dell 256GB M2 that is getting small. I'm also running a Dell SSD U.2 1,6TB 2,5" (MZ-WLL1T6A PM1725a) in the other flexbay. Moved the M2 drive between the bays to confirm that both bays work fine with PCI-e disks which it does.

I want to upgrade the bootable M2/U.2 to a larger drive.
Got an offer from Dell when I asked for an upgrade on the 256GB drive and I got recommended a 280GB U.2 Intel Optane 905P drive. Dont know if that drive is in stock any more. Tried a WD DC NVME 1,92TB U.2 drive and it did not work. Dell confirmed that the WD drive is not compatible with the 5820. My assumption is that Optane drives will work in my system.

Yes, I have read the manual but I need your experience on this.
Copy/paste from the manual:
Internally Accessible
• M.2 NVMe PCIe SSDs — Up to 4 x 1TB drives on 1 Dell Precision Ultra-Speed Drive Quad x16 cards
• Front Flex Bay M.2 NVMe PCIe SSDs -
• Up to 2x M.2/U.2 drives when Xeon W Series and Core X Cascade Lake CPUs are installed
NOTE: U.2 Optane memory is available with Xeon W Cascade Lake Series CPUs only.

To my questions:
-What is the largest PCIe /U.2 drive my system can accept as I already have an 1.6TB U.2? Will an additonal 1.5TB work?
-Will all Optane drives work; 905P-, P4800x- and P4801x-series?
-Drives I am considering:
*Intel Optane 905P U.2 2.5": 480GB or 960GB or 1.5TB
*Intel Optane DC P4800x U.2 2.5": 375GB or 750GB
*Intel Optane DC P4801x M2: 375GB

Thanks!