Not sure how something can be called a sandbox without the actual box part. As Siri is to AI, Flatpak is to sandboxes.
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.
My experience with android apps seems to be different. Every other app seems to be asking for contacts or calling or access to files.