dcabines

Try Webtop. It'll give you a literal Linux desktop in the browser and it works pretty well for me.

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.

Try bvckup2.com

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.

Transmission has been rock solid for me. Give it another try.