I mostly like your idea, but filesystems are probably the wrong abstraction for this - concurrency when writes are ongoing is difficult and corruption will like

I mostly like your idea, but filesystems are probably the wrong abstraction for this - concurrency when writes are ongoing is difficult and corruption will likely result. How about something like S3, overlaid with rclone's encryption: https://rclone.org/crypt https://rclone.org/crypt? The rest of the system could work like you described, exchanging keys via Matrix.