OpenMediaVault VS rclone

You might want to consider the previously mentioned MergerFS and SnapRAID, or MDADM and LVM for your setup. OpenMediaVault [https://www.openmediavault.org/ is a solid choice in this regard. Additionally, if you're planning to run something like Proxmox, you could look into deploying Starwind CVM on top of it. Pretty much like in this guide: https://www.starwindsoftware.com/resource-library/starwind-virtual-san-vsan-configuration-guide-for-proxmox-vsan-deployed-as-a-controller-virtual-machine-cvm/ .

You would better to install OMV instead. It's a ready to use fileserver solution with web interface.

So I'm working on a similar setup with an ancient desktop my in-laws were getting rid of. I installed OpenMediaVault directly to the hard drive (it's so old that I assume it wouldn't work too well as a hypervisor), with a 4TB external HDD attached. OMV supports Docker by means of a plug-in and I'm running a Jellyfin container with no issues at all. I'm still manually downloading everything to the HDD but *arr containers are my next step, as well as setting up a gluetun container to route all that through a VPN.

That is possible (it's automated to a certain extent). It also depends on the NAS OS. Unraid, for example, can work with drives of different sizes while using the largest one for parity. TrueNAS uses ZFS so you won't be able to just add a drive to your RAIDZ and expand it. There's also openmediavault: https://www.openmediavault.org/ that uses Linux Software RAID and you can add a new drive there but you can't combine drives of different sizes. And there's also Starwinds NAS that supports both Linux Software RAID and ZFS: https://www.starwindsoftware.com/san-and-nas. Plus, I would avoid SMR drives in RAID: https://nascompares.com/answer/list-of-wd-cmr-and-smr-hard-drives-hdd/

I'd put it like that: if you have a spare PC, then it's definitely worth trying. I would also look into openmediavault (minimal RAM needs): https://www.openmediavault.org/ and Starwinds NAS (supports both ZFS and Linux software RAID): https://www.starwindsoftware.com/san-and-nas

rclone: a command-line program to manage files on cloud storage.

Synchronise buckets with Sclone or Rclone

Says that Apple doesn't provide a multi platform API. It doesn't provide any official supported way to access iCloud from Windows, Linux.

There's a ticket covering everything you might ever want to know:

https://github.com/rclone/rclone/issues/1778

seconding rsync and syncthing.

the server could expose an smb or nfs share, the client could mount it, and then sync to that mount.

rsync over ssh also works, if you do not want to run smb/nfs.

this is also a cool tool https://rclone.org/

rclone [1] to dropbox. works since years without problems

[1] https://rclone.org/

rclone provides a special pCloud config option, which makes the setup straight forward. rclone can encrypt the data it uploads with its own encryption but not with the pCloud encryption. Therefore it can only upload data to the unencrypted pCloud folders, not to the Crypto Folder.

The arkclone project impliments rclone in ArkOS to achieve cloud saves. Not yet built in to ArkOS yet, and not a lot of recent traction on the pull request to get it added, but it can be installed manually.