However generally disaggregating storage makes HA simpler and allows for things like zero downtime patching: https://www.databricks.com/blog/zero-downtime-patching-lakeb...

Read replicas can be "shallow". You don't need to replicate all the data to create a replica. This allows to create them very very quickly (sub second).

All the extension still work. We don't support Citus today, but mostly because customers are not asking for it rather due to technical limitations. We support lots of extensions: https://docs.databricks.com/aws/en/oltp/projects/extensions