“properly power off the disk”, for an internal disk, isn’t really a thing. I don’t think that the OS, any OS, expects to regularly have disks totally lose power. Sure, hotswapping is a thing, but that’s still weird to do on a regular basis.

Having it “cold” to avoid malware and power surges is a fair idea, though. You’d still be concerned about the data lines getting a surge but that’s not as likely.

As a somewhat-less-cursed idea, how about an SBC like a raspberry pi on a smartplug inside your case? It can safely unmount the drive before asking the plug to turn itself off. Then there’s two devices that need to get crypto-lockered before you lose your data.

If you’re strictly just trying to keep the power down, configuring idle spindown for just that drive might accomplish the same thing.