> Btrfs [...] still doesn't match ZFS in features [...]
Isn't the feature in question (array expansion) precisely one which btrfs already had for a long time? Does ZFS have the opposite feature (shrinking the array), which AFAIK btrfs also already had for a long time?
(And there's one feature which is important to many, "being in the upstream Linux kernel", that ZFS most likely will never have.)
Raidz wasn't able to be expanded in place before this. You were able to add to a pool that included a raidz vdev, but that raidz vdev was immutable.
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.
ZFS also had expansion for a long time but it was offline expansion. I don't know if btrfs has also had online for a long time?
And shrinking no, that is a big missing feature in ZFS IMO. Understandable considering its heritage (large scale datacenters) but nevertheless an issue for home use.
But raidz is rock-solid. Btrfs' raid is not.