[rudasn]

Not similar to what I posted above, but similar to another idea I had but didn't pursue as much:

remote storage with local indexes, where the remote storage only holds encrypted data (can be postgres or s3 or whatever), and the only way to make sense of the data is if you have access to the local encryption keys and indexes.

The local indexes would function like your notion of tags, and also be somewhat encrypted (hashed probably) so you can find your data locally but only pull it from the remote when needed.