In my view there will be no solution you can expect from Databricks.

The main challenge is the shared handling of 

- metadata managed by Databricks in their account
VS
- your data being managed in your cloud storage.

So their (Databricks) design consideration (on UC-MS per region per account) turns out to be percieved as a design constraint for users of UC meta-stores as outlined very well by wojciech_jakubo and others.

Unfortunately the users need to work around this design "constraint".
A hierarchical namespace of catalogs can help but did not see this being mentioned in recent Data&AI summit.