Consider raid1c4 on gemini
BTRFS RAID56 is only so-so stable but raid1c4 is a very recent addition, too. Evaluate whether it might be safer to go with raid1c4. We should be able to reshape our current array on the fly if we do decide to change. I became skeptical when I raid https://lore.kernel.org/linux-btrfs/20200627032414.GX10769@hungrycats.org/ and https://lore.kernel.org/linux-btrfs/20200627030614.GW10769@hungrycats.org/.