TheHolm

If you need something which can withstand some bitrot on single drive, just use par2. As long is filesystem is readable, you can recover files even if bit of data get corrupted

It means you published 8080. Just stop doing it. nginx can reach that container via internal network (assuming they are on same network). Publishing docker-compose would help.

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

just a normal PC? Streaming should work in a browser.

Just a folder + syncthing. no extra infra is necessary + easy to backup.

With raid 10 - i would not risk it . With RAID6 ( obviously not on BTRFS) it is fair game if you have solid return policy for drives which are DOA. Go for SAS drives, they are cheaper (but generally hotter and nosier). And look for old-new-stock on specialized sites, no one in enterprise needs say 8tb drives, so they selling them cheap at times.

Get drive, connect , run long smart self-test ( for 18tb it is probably take a day). If it passed you are reasonably sure that it will not die soon. And keep running these test regularly, as soon as they start failing, replace.