It would be great to know the remaining disk space for the smallest device in a cluster (ie the one with the least remaining space). That way, I’d have some forewarning about how/when I might need to reorganize my data and devices/clusters as my usage grows.
The optimal UX might be something as basic as a file in the synced directory (maybe even at every folder level as a configurable option?) with a name that changes according to the remaining space. Eg
Named like that, it would pop to the top of a directory view (in GUI or CLI) sorted by name, or by modified date (but only when relevant), and thus would be brought to my attention, in context, at appropriate times (eg when I’m about to copy in a file).
Inside the file could be a relevant report with (at least) remaining space for all the devices syncing that folder.
Some may be bothered by that UX, so maybe opt-in?