SQL Server effectiveness of mirroring partial database
I think you should consider transactional replication instead of pulling data into a separate database and then mirror that database.

Replication in my opinion is not a high availability solution because for example it does not provide automatic failover, but I think it fits to your needs because you just want to replicate some tables and you can also apply filters to the published tables so you can replicate just part of the data and not the entire table (in case you want to do something like that). If you want to redirect applications to your replica, you could create a DNS entry pointing to the primary sql instance and set the connection string to use that DNS instead the servername or the ip and you just change the DNS to make it point to the principal instance or the replica so you don't have to touch your applications to failover.

Of course replication look at the limitations of replication and evaluate if this fits your needs.

As you mentioned, it is hard to provide a recommendation without knowing all the specs of your scenario, but I hope this can be an alternative to you situation.