I'm not the guy to ask this--I really dislike unRAID. But I'm reading this forum and you asked, so...I've had my morning coffee, here we go.
Before I continue, I LOVE your diagram and how it was done! I wish more people in the HL/HDC click would do decent designs like this. It helps to see what you have, and can help to show where you're going.
No. You should never downgrade. (unRAID fans, wait to flame, k?)
Proxmox is a real hypervisor, and they (the team there) keep adding useful features. ("Oh my I could have had a V8!" Wait, they do have a V8--Proxmox V8)
Proxmox supports LXC easily. I had never liked LXC then Proxmox servers forced me at ZFS-point to use them and damn they can be nice. Proxmox makes them look like a VM. Because they almost are in Linux.
Proxmox supports VMs. And it does it well. Sure you can do Debian 12 and with QEMU do the same--Proxmox is after all a layer over that, but it's a GOOD LAYER. It gives you control in a decent GUI. And my memory is horrible these days, but each Proxmox system looks the same. You run Docker in a VM, and you can run Kubernetes (RKE, K3s, K8s, whatever they make up next).
Proxmox clusters three or more systems together and easily supports shared (okay cloned) storage. You can move VMs much easier than multiple unRAID (I've only used two unRAID and tried to migrate from one to other, but I cannot imagine nine unRAID servers and trying to...really anything. My bias.
Proxmox gives you FREE backup--Proxmox Backup. Your friend failed to RTFM and see there existed an easy way to recover. (I'll point this out very graphically in a moment.)
Now, least I suffer the flames of the unRAIDians, I've used, supported, and TRIED to like (love? LOL no) unRAID. There are good things about it. And it DOES support ZFS now I understand. And container support? Seems very good. Let's talk about that damn sexy store attached to it--and you can add external stores--of apps (containers, Docker in this case).
I have six mini PCs with Intel and AMD chips, three old Intel NUCs, Dell XPS 8910, Dell XPS 8930, Dell PE T320-8bay, Dell PE T320-16bay, Dell PE T630 all running Proxmox in three clusters with shared storage in the clusters and all running backup as Proxmox Backup. And the three big servers (Dell PE) running TrueNAS Scale inside of Proxmox. Not my "real" NAS, but for some other uses.
The Dell XPS 8910 died and it took me 10 seconds to know it (notification system), and I recovered all containers, VMs to the Dell PE T320-8bay in less than 30 minutes. It was a cluster member, but still.
So I'm going to say do Proxmox right, and you'll grow faster with it.