How to do backups and maintan digital storage?

I've found the WD My Passport drives to be extremely unreliable and have had multiple fail over the years.

I stopped caring much if I lose my data, but do keep some important files (password managers, specific documents) mirrored. I find it easiest to use Syncthing to synchronize those important files across all of my devices.

For everything else I have an older computer set up as a server running NextCloud with no backup or failsafe. You could do something similar with FreedomBox.

I have several TBs of data to save (most space taken by photos and music), then I store in on a PC at home and I use samba (for local access) and sftp (for remote access) to access it. The computer has a single disk (it is a small form factor) and since there is no replication of all the data, I have another computer in another place and I configured borg, with borgmatic, to do a daily backup to it. The two computers are running Trisquel.

If the amount of data you have is small enough so that is is ok to be replicated on every computer you use, the easiest solution is syncthing, with a small deskop PC at home running permanently. With syncthing, there is nearly nothing to configure. For sftp, you need a way to make your computer accessible remotely, which may be a bit more complicated depending on your ISP.

Initially, I had a synology NAS with 5 disks and I tried putting 5 more disks on my D8. However, the power consumption is too high to leave it always on, and if for some reason the computer is not working, it is very difficult to find a computer that can accept 5 additional disks, so I concluded it was not a good idea at all.

--My server is not backed up. My laptop is not backed up. My external drive is not backed up.--
--I don't know how sufficient this is.--
I've been studying the subject for quite a few years. And data loss is inevitable. It's so inevitable that even the creators of the free/libre software DVDisaster will they you it's inevitable, but you can uses various methods to mitigate and even nullify that.

Overall it depends on you model.
For cold long terms storage the best of the best will be optical disc, and more specifically M-discs ISO/IEC 10995.
It's the kind of storage you burn once and you're set for life no matter where you live in the world. Anthropologists will love you for it.
https://www.backupwrapup.com/m-disc-founder-explains-how-it-keeps-data-for-1000-years/
Currently 25GB will cost you 4euros per m-disc for over 100 years of storage time.
So it's the cheapest solution of the average person.

Second is DVD gold plated from verbatim but those cost an arm and leg compared to m-discs.

Third short term/medium time cold storage is still optical medium with HTL bluray disc (around 10 years depending on specific weather conditions).

Fourth, tapes, financially inaccessible and many more problems thus only reserved for enterprises.

Then for anything else, be it HDDs/NAND, they must note be used to store data as cold storage, they have to be active. .
I've got enough experience to say that it's entirely to the warranty of the hardware and even then for what's considered high end I have seen failures.
I observed this over time and HDDs will have between 6000 hours and 30000 hours of lifetime.

HDDs at best have a 5 years warranty (30000) but data retention also have to be refreshed every fives years to be sure there's not alteration.
Aside the lifetime, you have to consider that reading the data of the platter is dependent on the electrical motor and magnetic driven head, two more possible failures, unlike optical discs which the reader is separated from where the data is stored.

NAND tech (ssd/usb etc...) is so tied to quantum mechanics effects that it's madness to store anything long term on it.
See the presentation "Why NAND Flash Breaks Down-VajB8vCsZ3s" on the subject.
And lets not talk about the DRM chips that each manufacturer adds to avoid "trade secrets" leaking and stops any possible recovery of data.

--And how much does all this cost?--
It all depends if you're talking about hot or cold storage.
Hot being one you need to regularly access or update.
Cold being backups/archives you look at when every now and then.

Eventually any part of hot storage becomes cold storage at some point.
Thus you need a proper redundant hot storage. To avoid potential loss of that buffer before it's sorted and becomes cold storage.

I'm going to consider that your model is a family one.
Anykind of computer with and hdd and a software to sync data between devices like lets say syncthing as well suggested by Avron.
Ideally you want at least three computers synchronizing the data over the internet securely.
If you have family members you can propose that so that each one host a server themselves at home so that every host a duplicate of each ones data.
The more you are the less probable you'll have data loss.

So in your case it's the cost of a computer that can run trisquel, plus a high end HDD or SSD (5 years warranty) for each member of the family that wants to joint the scheme.
Even old hardware from 2006 would do the trick, but you can also buy more recent hardware for 50bucks like those m710 thinkcenters or m710s ones they aren't very well cooled tho and heat does influence lifespan of hardware.

Then you go into cold storage as time goes on.
For cold storage get a M-disc burner, it's truly the only good solution long term for anyone.
Due to restructuring of many companies right now and also the AI bubble hoarding manufacturing resources on purposes it's hard and not cheap to get one but production will come back with time.
Then like I said currently 25GB of storage is right now 4euros and there's also double layer 50GB and triple layer 100GB which cost more per GB than the single layer 25GB.

I'll remind that the formula of the cost of storage is not cost per gigabyte, it's cost per gigabyte per lifetime of the storage.

I encourage you reading the manual of DVDisaster, it is superb, and it's a software available on Trisquel.
https://en.wikipedia.org/wiki/Dvdisaster
https://dvdisaster.jcea.es/downloads/manual.pdf

For a business it's not so different you'll just have a lot more redundancy like raid 5 etc... which is overkill for family settings.