Selfhosted

Thanks everyone, I feel much better about moving forward. I'm leaning towards Proxmox at this point because I could still run Windows as a VM while playing around and setting up a new drive pool. I'd like a setup that I can gradually upgrade because I don't often have a full day to dedicate to these matters.

MergerFS still seems like a good fit for my media pool, simply only to solve an issue where one media type is filling a whole drive as another sits at 50% capacity. I've lost this data before and it was easy to recover by way of my preferred backup method (private torrent tracker with paid freeleech). A parity drive with SnapRaid might be a nice stop gap. I don't think I feel confident enough with ZFS to potentially sacrifice uptime.

My dockers and server databases, however, are on a separate SSD that could benefit from ZFS. These files are backed up regularly so I can recover easily and I'd like as many failsafes as possible to protect myself. Having my Radarr database was indispensable when I lost a media drive a few weeks ago.

Proxmox is pretty much focused on ZFS, LXC containers and VMs. You want mergerFS and Docker. I say avoid Proxmox and go for Debian or another distro.

MergerFS and SnapRAID could be good for you. It's not immediate parity like with ZFS RAID (You run a regular cronjob to calculate RAID parity) but it supports mismatched drive sizes, expansion of the pool at any time, and some other features that should be good for a media server where live parity isn't critical.

Proxmox and TrueNAS are nice because they help manage ZFS and other remote management within a nice UI but really you can just use Debian with SSH and do the same stuff. DietPi has a few nice utilities on top of Debian (DDNS manager and CLI fstab utilities, for example)but not super necessary.

Personally I use TrueNAS but I also used DietPi/Debian for years and both have benefits and it really matters what your workflow is. OMV supports everything you want too (incouding SnapRAID) but takes extra setup which put me off.

Docker or LXC containers won't hurt your performance btw. There's supposedly some tiny overhead but both are designed to use the basic Linux system as much as possible: they're way faster than on WSL. For hardware acceleration it'll be deferred to the GPU for most things and there's lots of documentation to set it up. The best thing about docker is that every application is kept separate to eachother - updates can be done incrementally and rollbacks are possible too!

My server runs Debian VMs in Proxmox on an i7-2600 which has a lower benchmark than the 6600k. I also used the Perfect Media Server guide, and have 2 x 8T data drives pooled with MergerFS with 1 for snapraid parity, these are passed through to the main VM from Proxmox using 'qm set'. One thing I would often forget after deleting/restoring this VM was to run qm set again after restore, ensuring it has the flag to not back up those drives or else backups will fail and I have to go uncheck the backup option on each drive to fix it.

If I need to spin up another VM for tinkering it's easy enough to mount the NFS share as a volume with docker compose. Proxmox rarely shows CPU usage go above 50% (average is 10%) and this handles the whole *arr stack plus usenet and torrents in a single VM and compose file. I don't have GPU passthrough set up because the motherboard on this older rig didn't support IOMMU, never had issues with Plex or Jellyfin transcoding for Chromecast. I might build a new rig with GPU passthrough support to buffer media faster and selfhost LLMs when I get around to it.

I don't know about your first need ("MergerFS") but if you find useful, I have an old Intel NUC 6i3SYH (i3-6100U) with 16Gb RAM and I was running with Windows 10 for Plex+Arr and also HomeAssistant in VirtualBox. I was running into issues until I switched to Proxmox. Now I'm running Proxmox to run Docker with a bunch of containers (plex+arr and others) and also a virtual machine which has HomeAssistant and everything was smooth. I have to say that there is a learning curve, but it's very stable.

I use OpenMediaVault to run something similar. It’s a headless Debian distribution with web based config. Takes a bit of work but I like it.

Proxmox is Debian under the hood. It's just a qemu and lxc management interface.

Not calling you out specifically OP, but can someone tell me why this is a thing on the internet?

multiple 12GB drives

GB??? I assume TB automatically when people say this but it still is a speedbreaker when I'm thinking about the post.

None of your listed use cases will even come close to taxing the 6600k. It's going to probably sit happily in idle states most of the time.

Proxmox also has great snapshotting and backup features. Makes it easier to mess around with your containers/VMs without worrying too much.

Your CPU should be perfectly capable of that. I ran Proxmox with some VMs and containers on an i5-2400 with 16GB RAM just fine.

You could run on bare Debian as well but virtualization will give you more flexibility. If you get a Zigbee Dongle or the like, you can pass it through to the VM Home Assistant is running in.

I don't know MergeFS but usually the recommendation is ZFS.

Promox runs on debian. But anyway you will be surprised about proxmox can run in limited hardware. I have it running in a garbage mini PC and an old notebook :D

OP, I'm running Proxmox on and old Dell T320 /32gb RAM. I am not having any real issues doing so. I run Docker and a handful of Docker containers. I'm really not into the arr stack, but I wouldn't think you'd have much issue.

Proxmox is Debian. :-)
I do always suggest installing Debian first, and then installing Proxmox on top. This allows you to properly set up your disks, and networking as needed, as the Proxmox installer is a bit limited: https://pve.proxmox.com/wiki/Install_Proxmox_VE_on_Debian_12_Bookworm
Once you have it up and running, have a look at the CT Templates. There's a whole set of pre-configured templates from TurnkeyLinux (again, debian+) that make it trivial to set up all kinds of services in lightweight LXC Containers.
For Home Assistant a VM is your best bet, as it makes setting up connectivity way easier than messing with docker networking. It also allows easy USB passthrough, for things like ZWave/Zigbee/Bluetooth adapters.