If you set up a normal L2arc (read cache device) that will cache both data and metadata. However you can configure it to only cache one of the two. Set it to metadata only and size it appropriately and you have basically a read-only metadata mirror.

If you also want to have fast writes you can get a second SSD and set up a mirrored metadata device (storing metadata on mirrored SSDs, and regular data on whatever the rest of your pool uses)

Yes, the docs[1] tells you to match the redundancy level of the pool, but you could technically do something else.

Note the docs say level and not type. So if you have raid-z1 the a mirror would be appropriate as they both tolerate losing a single disk, and if you have raid-z2 you can use a 3-way mirror etc.

[1]: https://openzfs.github.io/openzfs-docs/man/master/7/zpoolcon...

You do, the normal way is to create a special metadata devices of mirrored SSD's. You do not need a 3 way mirror, as ZFS by default stores 2 copies of your metadata.