S3 isn’t sync and the rest don’t provide file systems or user sync. We built one at my previous company coupled to IPFS. The IPFS part isn’t reliable. So unfortunately there aren’t any that work - multi player, encrypted, and cross platform sync are the key requirements.

Fair. So it seems we need a multi-platform sync protocol which support user-owned storage (potentially one of those above).Fair. So it seems we need a multi-platform sync protocol which support user-owned storage (potentially one of those above).