Delightful Dude

Nextcloud is federated?

https://nextcloud.com/blog/federation-a-foundational-concept-for-digital-sovereignty/

You could do pull based backups with a central kopia instance and remote machines as described here: https://github.com/kopia/kopia/issues/1636#issuecomment-1596279322

Maybe Kopia is what you are looking for? It’s opensource and has a GUI:

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