I'd say you jump that bridge when you get to it. If you need something you just download it. Once you get stuck that you can't find it, if ever, you might look into getting a reader for your DVDs, or ask some friend or relative for a few minutes on their older computer.
Trying to redo a collection you had from some 20+ years old papers isn't worth it just to let it sit another 20 years.
Should be really easy to confirm just by doing a simple benchmark read on the device, probably a few GBs would do (more to make sure there aren't any RAM caching shenanigans).
That's probably good for one-off backups but Macrium can do:
Unfortunately Veeam doesn't do the last two, otherwise it's a nice contender.
It is extremely unlikely for the market to return to the sweet spot (price/TB) being at $50-$60 drives as it was before the "hard drive crisis that started at the end of 2011".
Beside being not that good at TB/$ the small drives will cost more over time in power, and taking more bays and crippling your upgrade possibilities, the chances to sell them for something when you need to upgrade and so on. Oh, plus nowadays you'll need to pay a premium to get out of the SMR doghouse, while with the large drives it just comes with the size.
In short just take one very large drive and wait for more money or increase (not much) the budget and get a similar one too. Or two of the medium-large-size. I'm sure you want 5 for redundancy, but this way you can do real backups (or even if you do RAID1 it'll be WAY safer than RAID5). No matter if you're losing "only" 20% with RAID5 versus 50% with RAID1 (or backups, much more recommended) it's probably more likely you can do 2x16TB in $300 than 5x4TBs (let's say either for 16TB usable). And better all around.
Sine wave is a newer marketing push
As a new development it's actually justified in a way, because this has been fueled by the Ukraine situation and the fear for brownouts and blackouts and most won't care about servers but about heating over winter. You might have the wood pellets, Diesel or even gas available but the heating system also needs some electricity, and sine wave for A/C motors (pumps, etc.).
You can get it with adb on non-rooted android: https://gist.github.com/ctrl-freak/24ac0e61b7cf550a6945
Literally less than 24h ago: https://www.reddit.com/r/DataHoarder/comments/17y8omd/small_offsite_backup/
It's literally why the site exists?!
That is kind of inconsequential as you can always compress the files individually if you wish and then make a tar with all of them together.
The question is what files you have, based on that various algorithms would do better or worse. And of course not doing solid archives would add a penalty to most algorithms if the files are somehow similar.
I'm not sure what you're expecting, the 2.5" drives are generally stuck in the past and underdeveloped, and all the large ones (and even the small-ish ones if they aren't old) are SMR, and their SMR is more perverse than the usual (or they generally just lack oomph and would crunch much longer and worse than their bigger cousins) and you want to use them with ZFS, and they've been already used a lot, and out of warranty.
Like the song says "I fought tougher men but I really can't remember when", it might be possible to be worse but I can't remember how (except maybe for the disks already throwing out errors).
I'd do directly Wireguard (this is what Tailscale uses but it's clear and controllable instead of more automagical). Openvpn or even directly openssh (of course configured with pubkeys) would be similar (run everything on non-standard ports to keep things quieter, and a non-standard user if applicable).
You can do local encryption there too, with LUKS, zfs, ecryptfs or even rclone (actually you can do it locally with rclone so the remote never sees cleartext).
Yea, I don't get this "race to the impossibly worse format". There were (even multiple) posts about saving wikipedia to pdf, I mean what for? The result will be impossibly larger and harder to use in any way as far as finding the article you want and navigating around, plus the format itself is just bad without reflow for reading on anything else except precisely one device of a certain (display/window) size and resolution.