I don't have a lot of experience in high speed but as soon as you start getting faster there tends to be exponential overhead. I think you should try mounting the network share on the Proxmox host to test speed without the complexity of the VMs. If you get the results you are looking for then you are good but if it is bottle necked there the bottle neck is on the NAS or SMB. SMB is particularly hard to overcome as it seems to be slow no matter what you do.