Resetting a new archiving database
The built-in way to reduce backup size for VLDBs with read-only data is to parition your large tables, placing older partitions on read-only filegroups, and then take infrequent backups of the read-only filegroups (or whole database), combined with regular backups of the read-write filegroups.  Then you can perform a Piecemeal Restore.


  Will [using UNION ALL views over multiple databases] work ?


Yes, you can typically replace a table with a view without changing the application, unless the application does something unusual.  The risk is in the performance, as the UNION ALL view across multiple databases doesn't really perform the same.

You might just try using columnstore compression on the large tables in the archive database.