Do you have backups? You mentioned family photos, so I hope so. If yes, have you tested them? Remember, RAID (or RAID-adjacent) is not a backup.

What filesystem are you using at the moment?

Questions aside, since you have precious photos, I would absolutely get a UPS. This will save you a lot of pain in the future, and you can continue to use it while you wait to upgrade. You can find them for relatively cheap on ebay, just make sure whatever you get has a new battery. Bonus points for a USB connection to gracefully shut down your raid array.

For the drives themselves, even if you aren’t shucking them yet (shucking meaning taking external drives out of their cases to put into a server), I would use shucks.top to find the best deals per TB. This comes with the upside of allowing you to shuck them in the future if you get a proper enclosure or chassis, and you don’t have to buy an extra case. Interestingly, external drives (like those listed on the website above) are generally cheaper than naked drives, especially when priced per TB. If you want to avoid shucking entirely, you can pick up DC rated drives for relatively good prices on serverpartdeals.com

The three main concerns with this setup, in my opinion, are power, heat, and speed. With raid, you’ll want the UPS as discussed, which covers the power issue. Since they’ll be in cases, it’s definitely possible they’ll heat up quickly, especially if you ever have to do a data rebuild or otherwise hammer them with lots of writes. As long as you’re able to keep them below 55-60C during those operations, you should be fine, but it’s something you’ll want to keep an eye on.

Apologies if that was kind of rambling. I’ve been a data hoarder archivist for over a decade. I’ve gone through several iterations of NAS and learned some hard lessons along the way. I encourage you to keep thinking about how to best secure your data from loss; it’s good to ask questions like this.