To stay roughly on-topic before I veer sharply off again: A week to expand a raidz seems acceptable, if it really took that long. That’s also roughly the time to do a full test / burn-in of a drive. So, new drive gets tested, 1 week, then expanded, another week - seems fine.
Free space display not being correct seems a larger annoyance than the time it takes to expand a raidz.
If the cost of acquiring the drive(s) weighs higher than the risk of needing to re-acquire all those media files, then no.
Risk tolerance is highly individual.
Lengthy musing on risk, skip as desired.
In my case, I too use the NAS for space-shifting media files. Everything exists on CDs, DVDs and Blurays. I am strongly opposed to using the -arr apps, “theft is wrong” kinda thing.
It took me probably weeks to rip everything to files - as in weeks of actual time spent, months to actually get it done. I don’t want to do so again. And because I don’t want to do so again, the extra parity drive is worth it. To me.
Space-shifting the media is a flaunting of the law I’m personally OK with, as I do own the physical discs and don’t share the electronic version.
I do not back it up, there’s my current risk tolerance. It’d be reasonable to build a second NAS as a backup target. But even that doesn’t get me to 3-2-1: This is too much stuff to keep an off-site copy, just by the sheer amount of time it takes to move all this crap over a 200Mbit up link.
Thinking further, if I lose the local NAS to fire/flood I also lose the physical media, and that means I need to re-acquire physical media (as well as shelter, clothing, and all my belongings). At that point I have other worries than whether my media library was backed up to cloud.
But I may talk to the husband and see whether he can stomach a second NAS as a send/receive target in the house.
Documents on PCs (not the media stuff) are backed up to cloud ofc, and to the NAS.