You can expand mirrored vedev based arrays, but you do it by adding vdevs, so you are looking at adding TWO drives at a time. You can also (sorta) expand Z1 arrays by adding whole additional vdevs, or by replacing each drive in the array one at a time.
As a point of reference, there’s a newer ZFS feature that will allow you to expand RAIDZ pools without creating new vdevs. Note, however, it doesn’t let you add or remove parity to change the RAIDZ level.
It’s rather new and will take some time for implementation into NAS platforms like TrueNAS.
the options i know are:
As it was said, you may try unRAID which would handle even the different size drives.
As alternative, it looks like mdadm should fit your requirements. You can use smth like openmediavault for that.
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