portable APT?

apt specifically isn't Essential, but it has a special case to treat itself as pseudo-Essential while making dependency calculations, to make sure you can't accidentally use apt to uninstall apt and then have no way to reinstall it. Non-apt package managers can uninstall it. (Try it in a minimal Debian container: "apt remove apt" fails unless you add some --force options, but "dpkg --remove apt" works fine, and presumably cupt would also allow removing it.)

Priority has very little practical effect, and Priority: required is mostly about ensuring that debootstrap and similar minimal-system bootstrap tools will install it, so that you have a route to install additional packages.