You can online expand a ZFS pool for years by replacing disks with larger ones or by adding vdevs. On current OpenZFS you can even expand a Raid-Z by adding disks to it.

To shrink a pool, you can remove vdevs. On OpenZFS this is limited to mirror vdevs, native ZFS from Oracle can remove Raid-Z.

Kernel integration is not so important as long as use distributions that include ZFS support at distribution level mainly Proxmox and TrueNAS or Ubuntu.