Selfhosted

For local backups it depends on what you want to have:

• The cheapest option is a usb or thumb drive. But you have to regularly plug it in and copy your backup on it.
• The lazy option is to buy a NAS and configure a backup job that regularly creates a backup. Versioned, incremental, differentials and full backups are possible as is WORM to add a bit of extra security. You can configure a NAS to only turn on specified times, do a backup and then turn off again. This will increase protection against encrypting malware. WORM also helps in this case.
  Or just let it run 24/7, create backups every hour and install extra services on it like AI powered image analysis to identify people and objects and let it automatically tag your photos. Cool stuff! Check out QNAP and Synology or build a NAS yourself.
  A NAS can also be configured to present its content in a LAN by itself. Any computer will automatically connect to it if the access isn't secured by user/password or certificate.

I recommend buying a NAS.

Thinking about this, the only ancient information we are still able to access is painted or edged on stone or clay. How about some sort huge wall with thousands of QRcode like engravings?

Just a hdd in usb caddy? IMHO good enough for 4 tier backup.

I'm thinking of using a HDD and keeping it at work, which is climate controlled. I'd bring it back every few months to sync the latest.

Since it's constantly being used, I'm pretty confident it'll be usable as a backup if my NAS fails, so it only needs to be "shelf stable" for a few months at a time. If you're retired or something, a safe deposit box at your local bank should do the trick.

After reading the previous discussion I think that you should get more than single drive to store cold backups. That way you can at least spread out the risk of single drive failing. 2TB spinning drives are pretty cheap today and if you have, for example, 4 of them, you can buy one now, write your backups to it and in 6 months buy another, write data on that and so on.

This way you'll have drives with year or two difference on purchase date, so it's pretty unlikely all of them fail at once and a single drive gets powered on and checked every other year or so. My personal experience is that spinning drives are pretty stable on the shelf, but I wouldn't rely on them for decades. And of course even with multiple drives you'll still want to replace them every 3-5 years each. Plus with multiple drives, if I were to build setup like that, I'd set up some sort of scripts or other solution where I can just plug the thing in and doubleclick an icon on desktop to refresh the data and maybe get a notification automatically that the drive you're using should be replaced.

And for actual, long term storage, printouts are the way to go. At least in here you can get books made out of photo paper with your pictures. That's one media which is actually stable over long period and using them doesn't require a lot of technical knowledge nor hardware. But I'd still keep digital copies around, as the printouts aren't resistant to things like house fire or water damage.

For photos? Archival prints. As a bonus, you also get a cool album to reminisce later in life.

This was a recent point of discussion on the 2.5 Admins podcast (https://2.5admins.com/2-5-admins-228/). Some good discussion on there.

My own thought is the best way to handle your family-member-finding-your-old-photos problem is the analog way: make some prints. It’s absolutely idiot proof, the methodology of keeping paper goods is well understood, and the technology is platform independent.

I usually use a dehydrator for ~3 days on my drives to make them shelf stable. So far I haven’t had any issues.