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FINALLY!

You can do borderline insane single-vdev setups like RAID-Z3 with 4 disks (3 Disks worth of redundancy) of the most expensive and highest density hard drives money can buy right now, for an initial effective space usage of 25% and then keep buying and expanding Disk by Disk, with the space demand growing, up to something like 12ish disks. Disk prices dropping as time goes on and a spread out failure chance with disks being added at different times.

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Yes but see my sibling comment.

When you expand your array, your existing data will not be stored any more efficiently.

To get the new parity/data ratios, you would have to force copies of the data and delete the old, inefficient versions, e.g. with something like this [1]

My personal take is that it's a much better idea to buy individual complete raid-z configurations and add new ones / replace old ones (disk by disk!) as you go.

[1] https://github.com/markusressel/zfs-inplace-rebalancing

I wish something like this would be build into ZFS, so snapshots and current access would not be broken.

True, but I have a gut feeling that a lot of these thorny issues would come up again:

https://github.com/openzfs/zfs/issues/3582

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