i ever use virtio driver for vm (windows and linux)....
i use stable driver version....
https://docs.fedoraproject.org/en-US/quick-docs/creating-windows-virtual-machines-using-virtio-drivers/
as i said, you have to configure multipath first......
https://pve.proxmox.com/wiki/ISCSI_Multipath
/dev/sdb and /dev/sdc is the same lun with different path.
when you correct add iscsi Lun (first step), then you have to create pv and vg on this lun.
Then you can add lvm.
don't check the option "use luns directly" when add iscsi lun....
you don't have to create a iscsi LUN, you don't have a iscsi SAN, but a fibre channel.
if your multipath is correct, you have a device named HPMSAecc under /dev/mapper/ .....
thuis is the device you have to use for configure LVM......
i think you have to use a redundant storage like san with two dedicated controller.....
or you can configure a cluster ha with nas4free (carp + hast)....
but with your setup, freenas is a SPOF as you write....
and it can't be clustered .......
glusterfs can be an other option...
from web gui of proxmox, Datacenter > storage > Add LVM ....
lvm thin can be used only for local storage, if I'm not mistaken.....
so use lvm, not lvm thin...
more info here:
https://pve.proxmox.com/wiki/LVM2