@Kevin Rossi Unfortunately hive_metastore can't be hidden as of now. It's not needed for UC, but a Databricks workspace doesn't work well without the default RDS connections which require changes in the way DBR/Spark starts up. Eventually we will have a UC-only workspace with no references to HMS, but that doesn't exist today. (Eng is working on it).
Here are the couple of things as. a workaround.
Configure the default catalog from hive_metastore to another catalog using "spark.databricks.sql.initial.catalog.name" property.
Default catalog can also be set while assigning the workspace to a metastore. If it's already assigned, unassign and reassign the workspace with a default catalog.
samples and system catalogs are read only catalogs, they can't be removed.
Regarding main catalog, we have a feature request called catalog to workspace binding. Be default, a catalog is bound to all workspaces, but using this feature we can bind the catalog only to the desired workspaces. In this case. If we disable all workspace access to main catalog, then it won't be visible on all workspaces. Please reach out to your Databricks contact to onboard your account to this feature.