> Mirroring across identical drives (from the same batch) seems like a recipe for trouble.
Yup. I've seen a couple of RAID5 setups collapse for precisely this reason. "Go to backup."
Suggest you look at ZFS rather than RAID. Step-change in what you can do, and serious step-change in reliability, at the bit-level.
Check out the 1-bit error examples in the photos there. (I lost 99% of 20yrs' photos to these errors, coming back from UK to Aus).
But note the ability to, eg: decide to upgrade OS, do Snapshot, Upgrade, discover nightmare, do RevertToSnapshot, and continue as-was In Under A Second without faff. (vastly better lowlevel approach than LVM, btw)