I've been looking into this, a lot.
Here is my experience so far. I'm sharing here just as much to keep my notes, but this is 2 weeks of videos and tinkering. I share this to give you a real sense of the challenge, it also separates the target customer base of unraid vs something like proxmox.
TLDR: proxmox is amazing because it's features are great, but generally orders of magniture more challenging to setup than unraid. unraid is what I went for in the end, because having something working is better than not, and meets most of my needs, oh, and saves me hours of time, I just click the GUI and it works, oh, and I can use my old HDDs however I like and they spin down saving £$£$ when not in use, which is most of the time. But you can install unraid inside proxmox to get what you want.
I started looking for 'free' products, so found proxmox to be a great solution for HA, storage and backup etc.. Watched many videos, thought it was 'the one' and off I went. Windows bye bye and proxmox installed.
Ok, I have friends with unraid, so immediatly expected similar functions.
I was wrong.
HA and Proxmox are really for super users, I mean, really super, I'm a dev, but don't want to tinker with this thing forever. I would like something to work.
Proxmox has some great features, but the simplicity at any level is not there. You can run many more services with less RAM and less CPU usage compared to Docker for example AND they will just magically migrate to another proxmox server if one fails. BUT, to get that working you'll need.... two servers, they don't use Unraid file system, so now you're at spinning disks spinning... all the time £$£$. There is no unraid, so you need 'real raid' lol, so now you're at what, Raid 5, performance is pants, ok, mirror. So now you need 2 disks just get somewhere. But you can't just 'add another drive'. You can if you use this LVM filesystem, oh man, I know more about file systems than I ever should (but I can see why the unraid crewe want to use ZFS now, lol). But if you just want to add another drive to LVM, get used to the CLI.
OK, so you've got your mirrored drive. But if you buy a new hard drive, well, you can't 'just add it', there is lots of work to do, but lets ignore this (this is clealry the killer feature of unraid)
Now you need that on another system
so, for 4TB of storage, you'll be using 16TB of disk space, that can't easily be expanded to just add that old 1GB disk you had laying around.
Oh, did I mention the GUI that is in Proxmox wants your mirror disks to be 'exactly' the same size, otherwise you're up for some pretty intense CLI sessions. Don't think you can just use a Seagate here and an WD there, oh no sir, they are 'slightly' every so *slightly* different sizes. hello cli.
OK, so you've got your 16TB of disks given 4TB of storage, you now need to setup ceph, which to be fair, looks quite simple, but I didn't get this far so that's from videos only.
Once you have this setup, you'll want a separate physical network for the servers to keep all the disks in perfect sync, and then the normal network for actual usage (e.g. connecting to the docker containers) - you can use the same one if you want, but, you're building a cluster here!
ahh, docker containers, well, you'll be installing a VM and then 'something' to host the docker containers, because proxmox dosn't support those nativly. It does support VMs and LXCs though, so you may find moving to LXCs much better, they are designed for proxmox and migrate really well (according to videos).
BUT, they are not the same level as sepration as docker, so there may be some security things to worry about.
There are actually many people that install unraid, inside proxmox to try and get the best of both worlds.
This might actually be a good solution for you.
You'll need a HBA card, it's a PCI Express card, with 1 or more 'mini SAS' connectors and you can pass through that to unraid, and a USB port for the key/boot.
Now you can have unraid, just add disks etc. and 'share back' to proxmox the files through the linux share system (NFS? not used it). Proxmox actually lets you 'add storage' like unraid, and it will appear for all the LXCs/VMs as a drive, but you can point it right at the unraid NFS share.
I assume you can tell proxmox 'don't auto migrate this (unraid) VM, as that will cause chaos' but for the odd LXC you could have that on, or even a different VM hosting dockers.
But you'll have say your databases on proxmox storage not unraid storage, since that won't be migrating to the new proxmox.
I hope this helps a little. and serves as a refrence for some.
In the end, I just installed unraid. and here i am :)