Where to start? Proxmox, Unraid, or TrueNAS

I have an old Lenovo system with a 4th gen i5 with 16 GB RAM and 2 8TB drives
I’m considering setting it up as a storage server.
I plan to eventually have a small cluster using something like Proxmox.

I’m looking for opinions on what to do with it.

One possibility is loading Proxmox on this system, and provisioning shared storage as the first service.
Or going with Unraid as I expect I will add disks to this system as I can, and not in large groups.
TrueNAS is an option, but I don’t think it will fit with my ability to only add disks occaisionally.

I prefer XCP-NG over proxmox.

I don’t have experience with Unraid or XCP-NG like Tom but I do have experience with TrueNAS and Proxmox, I don’t know if XCP-NG offers you a great way to expand storage by simply adding drives but I can tell you that Proxmox and TrueNAS definitely don’t cater to that market. If you’re absolutely set on adding single disks at a time you’re probably best off going with Unraid and running your services in Docker or VMs on top of that. From there if you decide you want a cluster I would keep the Unraid system as just a NAS and try out XCP-NG and Proxmox as a cluster on whatever machines you buy down the road and decide which you like. Personally I run Promox with all flash CEPH for VM disks with TrueNAS for bulk storage and am content to always add disks in sets, seems to be the way most of the industry has decided is the way forward and I don’t see any reason to fight their reasoning.

Not wedded to any particular solution at this point.

TrueNAS will be the storage solution for now. Key reason is that I was to leverage ZFS replication as part of the backup plan. Nothing else really fit nearly as well. The current system will have disks added in pairs as no other method makes sense for a build using OEM hardware.

Next thing I want to do is get a second system running to use as a replication target. Off-site will eventually be a third system at my father’s place in Oklahoma.

I will be experimenting with virtualization, but I want a solid storage framework in place to support it.