

I wouldn’t add a new glass sheet + magnetic sheet + PEI sheet on top of the original bed on a bed slinger because this is probably at least twice the weight of stock, which will cause issues when you get it moving back and forth rapidly. Think of how agile a petite individual can be compared to someone the same height who weighs 300lbs.
Z1 is just single parity.
AFAIK expanding a ZFS pool is a new feature. Its used in Proxmox but their version hasn’t been updated yet, so I don’t have the ability to try it out yet. It t should be available to you otherwise.
Sweet build! I have all these parts laying around so this would be a fun project. Please share your enclosure design if you’d like!