Twitter ran into the same problem. They solved it in a different way. They merged approach #1 & #2 described in the article. For high fan out, you don't write to the inbox of all their followers. This avoids high write amplification when a popular celebrity tweets. Instead you send notifications and have the followers pull tweets from popular celebrities while constructing their timeline.

Having worked with Cassandra & Redis extensively, this might work better with Cassandra. Cassandra already has sets, maps, list as datatypes and can be leveraged to build a CRDT without having to write a new database. Oh, well I'm guessing we're past that conversation? ;)