The problem is more with zfs on consumer grade NVMes. I have/had problems in that configuration due to the bigger sector sizes. Proxmox itself does do frequent writes, but I don't know how often exactly. I know that my problems went away with not using zfs.
https://www.reddit.com/r/Proxmox/comments/idlqh3/zfs_extremely_high_ssd_wearout_seemingly_random/
I wouldn't suggest usb or sd-cards with proxmox due to its constant logging. You will fry them really quick unfortunately. Had that problem with NVMes.
For litterly anything else I would also suggest SD-Cards.
Hey, i have had the same trouble on an DL380 G9. Those bioses don't support booting from PCIe at all. My server can't even boot from drives from the Raid controller in IT-Mode.
I would suggest, by proxmox being a hypervisor, to just install proxmox on a single SATA disk and try to boot from there. This is what I have done in the end.
You can then use your NVMes as storage pool. Also you bifurcation can always also be a problem when trying to boot from those devices.
I would also as a last call try to disable bifurcation and see if one drive will show up. Maybe then you could use 2 real PCIe slots with cheap m2 to PCIe adapters.
I suspect nextcloud having performance issues with slow Disk IO. With rootless containers I had a much worse performance than rootfull. Also using MySQL Backend instead of SQLite did speedup the performance.
Nevertheless I have the same problems with nextcloud as you stated. Pretty much not as usable as I thought.
Does it really matter? I mean you can't be faster than light, wich is around 300km/ms which we pretty much are. I see this more as a bandwidth type of improvement. In theory we could do 65ms around half the globe with a diffct fiber connection and about 900 trillion watts of energy but thats not really the use case I think.
A better improvement would be WiFi and 5g stability and latency. I loose more latency over WiFi than over my entire connection to any server I need.