Been lurking on this thread for a bit.
RE: bootable USB issues - I tried loading ESXi 7.0.3 but got the "esxi fatal error 8 device error" message.
Disabling USB 3.0 in the BIOS (from AUTO to DISABLED) partially solved the boot error for me. It is hit or miss on reboots but my hypervisors stay running 24x7 once booted. Occasionally when doing updates I'll get a boot error but bouncing the server and it goes away.
RE: fans. If you find an HPE reseller (like Avexon, LLC) you can place an order for an OEM fan replacement. I had to replace a fan on mine and ordered one through them. Works great.
RE: RAM. Part numbers 809083-091 805351-B21 (search eBay) will get you squared away. I bought two sticks of 32GB for a total of 64GB ECC. Works without issue.
RE: expandable, external HDD device. You are looking for HPE part number 861161-B21. 4x bays. ec200a will mount on top of it.
RE: NVME comparability. M2 2280. No issues with Seagate Barracude Q5 1TB or 2TB QLC drives. I tried an "off brand" 2TB NAND and it died within a week. Wondering if heat is/was an issue. The Seagates seem to tolerate more.
RE: RAID. The embedded SATA controller will support software-only RAID 0/1. RAID 5, RAID 6, RAID 1+0 is out because you are limited to just two physical disks (excluding NVME). I haven't tried RAID configurations on this device as I am using the disks for datastores.
support.hpe.com