I dont know about mtls, but Obsidian with the Self-Hosted Live Sync community plugin has end to end encrypted sync between any device. There are a ton of plugins to make it capable of doing whatever you want. I have it syncing between Windows, Linux, and Android currently.
You seem to be under the impression that the “buckets” in this case are all or nothing. They are talking about partitioning the drives and raiding the partitions. The way he describes slowly moving data to an ever increasing raid array would most certainly work, as it is not all or nothing. These buckets have fully separate independent chambers in them that are adjustable at will. Makes leveling them possible, just tedious and risky.