Citus is great but it's just sharding. Only some very specific use cases fit within those limits, and there are cost issues too (being ha requires replicas of each node etc). RAC masters don't need to sync writes to each other, that's not how it works. Every node can be writing independently, they only communicate when one node has exclusive ownership over a block another node needs to write to, and the transfer then occurs peer to peer over rdma. But if writes are scattered they work independently.

> they only communicate when one node has exclusive ownership over a block another node needs to write to

and it needs to communicate that he has exclusive ownership, and also after each writes you need to invalidate cached data on other nodes.