I feel like there's some room for improvement.
You're not going to prevent silent bitrot no matter what modern technology you use, so take a proactive approach instead to prevent data loss.
However, I found that I had a lot of data to back up, and it was actually cheaper and less tedious to get 4TB USB hard drives for ~£100 each, and plug them into an old defunct EeePC901 (with the added advantage that if the power goes out it has a battery).
My main PC has an SSH private key that lets it access a restricted shell on the EeePC that only allows it to give it files to store. That way, if a hacker breaks into my internet-facing machine, all they can do to my backups is fill the disc up, not delete or access anything. I have a process on the EeePC that regularly scrubs the par2 files, and the hard drives (I have two so far) are formatted with BTRFS, so given all the data is regularly read by the scrub process, that should notice any drive failures. My main PC uses ZFS, so I have safety in variety.
I also have an off-site backup stored on an encrypted USB hard drive in my locked locker at work, which isn't updated as regularly. My internet connection is slow, so I use the rsync --only-write-batch trick, and then carry the large update file to the backup on my laptop.
What could possibly go wrong?
If you're going for 3.5" drives, then yes I can well believe that the sweet spot is with slightly larger drives, especially if you take enclosures into account. I did the calculations for work a while back for shoving hard drives into something from https://www.45drives.com/ and it seemed that getting the largest drives possible was the best price/capacity option.
Something that's easy to overlook with larger drives is that their rebuild times are worse.
"Shucking" drives throws the economics way off even if it means having to do some hacks and losing warranty... Usually the drives that come in enclosures are smaller.
A lot of ways to be efficient with money start by having or using a lot of it:)
You need to keep them spinning on a regular basis, and replace them as they begin to fail.
This is usually a precursor to SMART errors happening in the near future, but unfortunately, it can still result in corrupted replication and corrupted backups; as your backups would be backing up the rotten (corrupt) data.
I've witnessed this happen on both Seagate and WD drives, on systems with ECC memory. I can only suspect this is due to HDD manufacturers wanting to reduce their error rates, and RMA rates: it may happen with the ECC bits in a sector is corrupt, making bitrot undetectable. Instead of giving an error (and being grounds for a RMA replacement), the HDD firmware may choose to return non-integrity-checked data; which would usually be correct but also could be corrupt.
It's why filesystems like ZFS and btrfs are so important.
My rough estimation of this, based on my own experiences and those on r/DataHoarder, suggests 1 hardware sector (4KB for most drives post 2011) will silently corrupt per 10TB year. Such corruption can be detected via checksumming disc systems like ZFS.
Usually, the whole sector is garbage, which is not indicative of cosmic ray bitflips.
External flash memory storage like USB sticks and SD cards fare far worse. In my own experience, silent corruption occurs more like 1 file per device, per 2-3 years; irrespective of the size of memory. I've had USB sticks and SD cards return bogus data without errors, so often. I only know because I checksum everything, otherwise I would have thought the artefacts in my videos or photos came with the source.
If, in 2020, you are not using ZFS or btrfs for long term archival, you are doing something wrong.
ext4, NTFS, APFS, etc may be tried and tested, but they have no checksumming, and that is a problem.
However, at work, I have backed up ~200TB of data to a large server with RAID-6 and ext4, storing the backups as large .tar files with par2 checksums and recovery data, and regularly scrubbing the par2 data. I have yet to see any corruption whatsoever. These are enterprise-grade hard drives. This is the strongest evidence I have yet seen that the enterprise-grade drives are actually better than the consumer-grade ones, rather than just being re-badged.
I should really get around to converting the main drive to btrfs, but this works well.
Much like other commenters I'm no expert on the topic, but I think you'd have to be pretty incredibly unlucky to have a mechanical failure on 3 drives at once from lack of use, especially if they were from different manufacturing batches.
Now get Carbonite (not affiliated, I just like the infinite space backup), and get it to backup your key laptop folders (Docs, Images, Desktop, etc) and your L-drive.
I don't remember how much it costs ($6-10?/mo), but I have stopped worrying since then. I got a monthly tib file for my system and an "instant" backup for everything else. So even if my laptop is stolen I can set up a new laptop (the .tib may be useless but I can open it and see what s/w I had and I can take the config files/folders to move to my new system).
I don't remember how much the disk was but it didn't hurt my wallet, and the ~$100 (?) per year on Carbonite (had CrashPlan) definitely doesn't hurt my wallet.
If you do all these things, I think that's about the best you can do with optical media.
There might be a better medium available nowadays but if I seriously wanted to have a piece of data fifty years from now that's where I'd start.
There are LOTS of failure cases with any cloud provider, especially one with a crazy policy of deleting data in just 45 days.
There is at least 1 reddit post a month about how someone lost data with Backblaze. Their reddit support rep is never able to do anything about it, other than "sorry, we will take on board your feedback".
For comparison, if your Google Drive subscription lapses, Google stops you from uploading but will not delete your data.
A good lesson was learned but it hurt. The upload took weeks to complete.