I’ve been using fwup (https://github.com/fwup-home/fwup) in embedded Linux consumer products for almost 10 years and it has Just Worked.

Supports A/B updates, uboot integration, and signed update files.

Not sure about buildroot, but in Yocto it’s fairly straightforward to write an image creation recipe that bundles your ext4 filesystem image(s) into an update file you can deploy.