If visits expire after 30 min, why not rotate the salt every 30 min? Keep current and previous salt, update as needed.

I would have more faith in privacy, if you didn't store the salt in the DB or permanent storage. If you manage to statically load-balance the users (e.g. hash site, ip, user-agent, don't forget site), the hash could be in-memory only. Sessions would break on server restart, but that's more of a feature.

To move thing further, you might not even need to store the hashes in the DB. Keep them in server memory only and (real-time) update aggregate data in DB.