To be honest I don’t find it all that inflexible.
When you consider that you’re limited by hardware anyway, you plan your ZFS array ahead of time like I did.
As the years go on and drives fail you eventually replaces all disks with slightly larger ones.
@prologic@twtxt.net how old is the actual appliance? You can replace disks all you want/need, but the life of the appliance itself is finite.