Shelly Plugs, although I haven't yet set up any local data gathering, just using their cloud stuff.

ZFS is the gold standard. Since it's a dedicated NAS, I'd say just run TrueNAS CORE.

While it's not perfect, vdev expansion was merged recently and will arrive in OpenZFS 2.3

As for how to configure the stuff... Depending on your use case (which you don't explain much) and how often you expand, I'd probably go with just a plain mirror of those 12TB drives. ZFS does allow you to add more vdevs to a pool, so that's a non issue. Once you fill that one mirror, you can add a second mirror, and still have that as a single pool.