I've used RAUC (https://rauc.io/) professionally for a couple of projects and am happy with it. There's a RAUC meta layer which provides bbclasses for generating rauc bundles from an image recipe. It's not that complicated to set up boot partition selection in u-boot.

For embedded systems, I strongly prefer the "full immutable system image update" approach over the "update individual packages with a package manager" approach. Plus you get rollbacks "for free": if the system doesn't boot into the new image, it automatically falls back to booting into the previous image.