I'm not sure the community is looking for something like that, but for what it's worth, here is what Proxmox is proposing for ZFS capable SAN appliances: https://pve.proxmox.com/wiki/Storage:_ZFS_over_ISCSI

AFAIU, it's a storage class based on a framework using ssh to create ZVOL and share them over iSCSI on a SAN appliance. Storage snapshots are then triggered on the SAN.
And it's apparently possible to provide your own SAN implementation as a plugin.

Sounds like part of a solution to me, but I have no idea whether it would be suitable/desirable for XCP-NG.

Aren't typically the snapshots done on the SAN side in these cases but need to be driven by XOA ?

No. By default, everything is done by XCP-ng. The good side is that regardless the storage vendor, it will work. That's a strength, to be agnostic on the hardware. The price to pay is to rely less on the vendor capabilities. We have plans this year to start with some storage vendors to offload snaps and other things, but as you can imagine, since there are hundreds of different vendors with hundreds of different models, you cannot have something universal.

Aren't there any backup integration existing for backup with SR on SAN?
Aren't typically the snapshots done on the SAN side in these cases but need to be driven by XOA ?

I thought it would be a fairly common use case for enterprise setup.

My personal use case though, is to deploy VMs on an XCP-NG instance deployed on a dedicated baremetal server and use SR from SAN/NAS storage (iSCSI & NFS/SMB) implemented by a dedicated TrueNAS server instance.

I'm not sure to entirely get the use case, but IIRC you have web hooks for backup jobs (pre and post).