Snap and Flatpak do both sandboxing and package management.

You can use the underlying sandboxing with bwrap. A good alternative is firejail. They are quite easy to use.

I prefer to centralize package management to my distro, but I value their sandboxing efforts.

Personally, I think it's time to take sandboxing seriously. Supply chain attacks keep happening. Defense is depth is the way.

Notoriously not actually secure, at least in the case of Flatpak. (Can't speak to Snap)

Not sure how something can be called a sandbox without the actual box part. As Siri is to AI, Flatpak is to sandboxes.