Either pay a cloud storage provider about $4/month to hold it for you, or buy a couple external drives and verify your backups yourself about once a year then replace drives as they eventually fail.
I figure the break even point is about two and a half years. If you’re committed to hoarding your content for longer than that, buy storage drives. If not, rent cloud storage. If your drives die in less than that you would have been better with cloud storage, but that isn’t likely.
Seafile has SeaDrive which mounts like an external drive and works just like other similar services like OneDrive.
A mirror is not a backup, like you’ve noticed. Use rsync for your backups if you don’t care about versioning or snapshots. You can run it on a schedule to copy files to your backup, but not delete anything. Then you can manually run it with the delete option turned on when you’re ready.
I like D2 because it is declarative and I can keep the model in source control.
Dozzle may be a bit basic for your needs, but it could be one tool in your toolbox.
If you don’t need it to be on your network and you don’t need it to be on 24/7 and you don’t need any running server services why not use a larger external enclosure? You could even get a 10 bay.
I like my SFF paired with externals for a not-always-powered-on archive and backup. I don’t think I’ll build a big tower PC ever again.
No warranty = no purchase.
While the par file suggestions are the most reasonable thing to do there are some more interesting alternatives.
Try horcrux. It’ll split your file into several pieces and make it so you only need a few of the pieces to recreate your file. You could make 99 pieces and only require 3 of them to reassemble. That way if most of them are damaged somehow you can still recreate your file.
This is similar to how Storj splits your files into 80 pieces and only needs 29 to recreate your data. It is also similar to how satellites transmit data when part of the message can be lost in transmission.
I like to prefer images from a known source like linuxserver.io.
https://silverbullet.md/ and I may start using https://d2lang.com/ too.
Alternatively, https://syncthing.net has to run as a service on both machines, but it is a great tool.
Transmission has been rock solid for me. Give it another try.
That and https://filebrowser.org is free.