Well, yes. You can't just convert filesystems in place, whether ZFS or otherwise. After installing the package you can create ZFS pools and filesystems. If you want to replace existing filesystems you would move all your stuff over after the fact.
Booting from ZFS will require some extra steps. I actually cheated here and just left /boot and / on ext4, using ZFS for /usr, /home, /var and everything else, so I don't have firsthand experience with this (on Linux anyway, on FreeBSD you can just select ZFS during installation). But if you do want that, it doesn't seem too difficult, https://openzfs.github.io/openzfs-docs/Getting%20Started/Deb...
As far as ext4 checksumming, as your own link says, it only checksums metadata -- actually, a subset of the metadata apparently. ZFS checksums everything, meaning your actual data. Further, since ext4 is not aware of redundancy -- you'd be using lvm or mdraid for that -- it's not clear what it can do if it does detect an error. With ZFS these errors are fixed automatically.
Booting from ZFS will require some extra steps. I actually cheated here and just left /boot and / on ext4, using ZFS for /usr, /home, /var and everything else, so I don't have firsthand experience with this (on Linux anyway, on FreeBSD you can just select ZFS during installation). But if you do want that, it doesn't seem too difficult, https://openzfs.github.io/openzfs-docs/Getting%20Started/Deb...
As far as ext4 checksumming, as your own link says, it only checksums metadata -- actually, a subset of the metadata apparently. ZFS checksums everything, meaning your actual data. Further, since ext4 is not aware of redundancy -- you'd be using lvm or mdraid for that -- it's not clear what it can do if it does detect an error. With ZFS these errors are fixed automatically.