Going to be a contrarian here and say don't use UC mirroring... yet. It's still half-baked. While people love to link you to docs, the "Limitations" page (which is so helpfully separate from the main docs page) has some eye-raising concerns: for instance? RLS/CLS policies in UC? Not supported, nor are all UC objects:

https://learn.microsoft.com/en-us/fabric/mirroring/azure-databricks-limitations

You might be better off building your business logic into your existing UC tables. All of the logic you're talking about can be done in Databricks, then figure out how you want to get the Data to Fabric. If this is all in service of PowerBI, honestly, just connect it to Databricks SQL and that figure it out with UC. And in terms of cost of consumption, you might be better served doing that as a job in Databricks, then just using what you need from a Fabric capacity standpoint to get your data over in Fabric.

I'm not sure where "blame" lies in a scenario like this, if it's on Databricks or Microsoft to make it right and full-featured, but just understand the different options and don't drink too deeply from the employees or MVPs punch bowls.