You don't need Flatpak on FreeBSD. Flatpak was created for a problem that doesn't exist on FreeBSD. There are not a thousand different FreeBSDs with varying kernels and software installed.
I miss being young, having lots of energy, being able to function on 4 hours of sleep and having all my life in front of me as much as the next person but you'll have to take systemd, networkmanager, docker/kubernetes and zfs off my cold, dead hands.
I thought zfs came from Solaris/FreeBSD
- zfs came from solaris, not from freebsd
- solaris and freebsd share very little as codebases and are very different OSes
- and regarding the previous point: the fact that some OSes shared the codebase ~30 years ago means very little today.
- the zfs people use on FreeBSD today is basically ZfsOnLinux. Som years ago the freebsd people dropped their snowflake implementation and joined the ZfsOnLinux effort, basically because ZfsOnLinux was moving at a way faster pace than they were. The project has been renamed OpenZFS iirc, and FreeBSD periodically rebases on that afaik.