Need to have an off-site backup solution as well, and cloud storage is convenient.
Some options, with prices in USD:

Some tips for zero-knowledge backups.

"May be more cost-effective to deploy hardware at another physical location - to check electricity usage."

Link: https://rclone.org/

Strengths:

• Supports crypt which acts as an additional encryption layer to sync to remotes. According to Hacker News, seems trustworthy and reliable. Will be useful for data storage on untrusted environments.
• Supports many cloud, local and remote backends.

Downloading rclone is quite straightforward, using curl https://rclone.org/install.sh | sudo bash which checks for the current version of rclone. Note that the apt repositories for rclone are currently stuck at version 1.50.2-2.

Need to first configure remotes with rclone config, during which the sync backend can be specified. Gave a hand at sftp. Attempted to perform a backup of the Dokuwiki directory, yielding a SFTP failure. See error messages below:

$ rclone sync -P /dokuwiki/ nas:/dokuwiki/
...
2021-08-09 22:33:51 ERROR : data/cache/a/a41386c9523185547b948bab29f8724f.repo: Failed to copy: Update ReadFrom failed: sftp: "Failure" (SSH_FX_FAILURE)
2021-08-09 22:33:51 ERROR : data/cache/a/a4146da96d57160590dfd1faae419088.media.600x231.png: Failed to copy: Update ReadFrom failed: sftp: "Failure" (SSH_FX_FAILURE)
2021-08-09 22:33:51 ERROR : data/cache/a/a449e6e40ea628ef213ea69daa88689e.media.90x67.png: Failed to copy: Update ReadFrom failed: sftp: "Failure" (SSH_FX_FAILURE)
2021-08-09 22:33:51 ERROR : data/cache/a/a45e71078c542b6f00dd1b05d4741d48.repo: Failed to copy: Update ReadFrom failed: sftp: "Failure" (SSH_FX_FAILURE)
Transferred:       29.820Mi / 88.080 MiByte, 34%, 6.867 MiByte/s, ETA 8s
Errors:               735 (retrying may help)
Checks:              3925 / 3925, 100%
Transferred:            1 / 6088, 0%
Elapsed time:         4.0s
Transferring:
 * data/cache/a/a128707d9…dia.4032x2240.crop.jpg:100% /745.498Ki, 0/s, -
 * data/cache/a/a184ab518…a0e981e0d8fb0.metadata:  0% /10, 0/s, -
 * data/cache/a/a4146da96…dia.2867x1083.crop.png:100% /201.676Ki, 0/s, -
 * data/cache/a/a45e71078…f00dd1b05d4741d48.repo:100% /1.497Ki, 0/s, -^C

Since SFTP transfer is the primary go-to for low cost backup (see the full list of available backends), rclone is currently not effective at the moment.

Some drawbacks of rclone include: (1) no snapshot-based backup. Note this idiomatic command.

Strengths:

• Fundamentally simple at its core: rsync algorithm written in Python.
• Designed to work over shaky network connections, as opposed to regular scp. Supports transfer resumption.
• Often recommended over scp, and has a verification check, see SO for a comparison.
• Highly scalable.

Possible strategies:

• https://superuser.com/questions/153176/how-to-rsync-a-large-file-with-as-little-cpu-and-bandwidth-expense-as-possible
• https://opensource.com/article/19/5/advanced-rsync
• rysnc -avhlAXEWP --itemize-changes --stats --info=progress2 /source/ /destination
• https://www.reddit.com/r/selfhosted/comments/wlj923/comment/isnko5x/

• Guide: https://ostechnix.com/setup-backup-server-using-rsnapshot-linux/

• Scalability of Kopia: https://www.kasten.io/kubernetes/resources/blog/benchmarking-kopia-architecture-scale-and-performance

• Repository: https://projects.torsion.org/borgmatic-collective/borgmatic

Link: https://restic.net/ / https://github.com/restic/restic

Main drawback seems to be its inability to safely handle very large datasets (GB~PB), as well as memory usage and file corruption.

Strengths:

• Deduplication

Incremental dumps using tar: https://www.gnu.org/software/tar/manual/html_node/Incremental-Dumps.html

Looks like Duplicity doesn't support backups from remote: https://serverfault.com/questions/543039/backup-a-remote-directory-with-duplicity

duplicity /volume1/local/test scp://backup//volume1/remote/test

• Encrypted backup strategy...? Delta backup of encrypted data
• Borg
• Alternative workflows? others

Wrestling with Kopia for a while now... The full keyfile and known hosts need to be supplied via command line (why?), and in the end the following error appeared: ERROR can't connect to storage: unable to open SFTP storage: error establishing connecting: unable to create sftp client: ssh: subsystem request failed