I added the following to a previous message, but you might not see it. Partitioning your 1.6's into 0.3+1.25's, also means that metadata and small.files not only can't crowd each other out, but they can be independently.migrated to larger devices over time, if needed.
So for example, say your small files grew. In a years time you could buy 3 x 512 GB SSDs for metadata only, zpool attach them to the metadata vdev and let them resilver, then zpool detach all 3 original 300 GB partitions. Then, next stage, you zpool detach a partitioned 1.6 from the small.files vdev, unpartition it, and zpool attach it back. Repeat x 3 and when the last of the 3 is attached back, autoexpand will kick in and the small files vdev will occupy the entire 3 x 1.6 mirror, not just 1.25 of it.
But bear in mind, you can only grow vdevs that way, not shrink them, and with Z3 you can't remove a top level ssd vdev, only grow it or (by growing to a new device) migrate it bigger. You can't zpool remove a top level vdev with Z3 in the pool - that's a ZFS limitation - so whatever you do, if you evaluate your needs and you need more ssd space, switching mirror disks to slightly larger mirror disks is the only option, apart from destroy and rebuild.
For that reason I'd keep the original partitioning to under 250 GB for metadata. Its probably plenty, and means you can later migrate metadata to 3x 256 not 3x512 which will save future money. To.do this, you could partition them all something like 250 GB metadata (nvd1p1 nvd2p1 and nvd3p1) + 900 GB small files (nvd1p2 nvd2p2 and nvd3p2) + 425 ish GB freebsd-swap on each ssd (nvd1p3 nvd2p3 and nvd3p3). Then as time goes on, you can see how both vdevs fill, and you can easily one ssd at a time detach, reformat to say 400+900+275 swap (if you need more metadata space), or 250+1100+225 swap (if you need more small file space), reattach the ssd, and let zfs autoexpand the required vdev selectively that way. Later on you can repeat and expand again, and then again later, using more of the swap space you held back, until the ssd is fully enough used (70-80%?), and the 1.6 ssds can't hold both metadata and small files and you need to get more ssds or let some small data sit in the main pool. That way you don't reserve sizes that your pool doesn't really need, waste space, and force an avoidable extra purchase or larger purchase down the line.