The Portable Freeware Collection Forums

Kopia open source backup system. Looks very simple with lots of functionality.

Cross-platform backup tool for Windows, macOS & Linux with fast, incremental backups, client-side end-to-end encryption, compression and data deduplication

Status: Not portable. I haven't tested this yet but I'd be very surprised based on what I'm seeing here: https://github.com/kopia/kopia/issues/432 (thanks to badrelmers whoever you are). Appears to be Electron-based, but there's a CLI app as well that might work for your needs.

Home page
https://github.com/kopia/kopia

Download Kopia 0.9.3
https://www.softpedia.com/get/System/Ba ... opia.shtml

Discussion about Kopia: Encrypted Backup Shootout on Hacker News
https://news.ycombinator.com/item?id=25618346