Hi @TVKP , welcome to the forum.

All your questions are valid to ask. The answers, as usual, depend on variables that are too many to list them all.

1 Storage: the choices between centralized SAN and Distributed storage will be influenced by: your expertise, your comfort with technology, your need/want of reusing existing infrastructure, your budget, etc. There is no simple "this is better than that'.

2 Backup: Your tried to point what backup solution at iSCSI target that is presented as what?
Lets assume for a second that your tried to use PBS and point it to initialize an iSCSI disk. Last time I checked the PBS specifically filtered out iSCSI LUNs. You can certainly workaround that by preparing the DISK/FS manually.

3 Cluster: the cluster discussion is a weekly occurrence in the forum. There are several long and informative threads. Asking a specific technical question will be more productive than simply asking for suggestions. That said, I suggest starting with https://pve.proxmox.com/wiki/Cluster_Manager

4 You are likely not thinking that you need a Partner but you may actually benefit from one: https://proxmox.com/en/partners/find-partner/explore

Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox