If mongos could have any parameter like read only from secondary shard nodes.
We could use secondary preferred or secondary read preferences, but in that case any user might not use this preference and able to directly read from Primary and if one need to restrict that mongos complrity from reading primary, not found any.
To stop this we have to create a mirror cluster that will sync delta, for that we can use mongo-connector or mongo-shake third party tools.
But these also come with challenges like: Index sync is not possible.
Here is community discussion I had: https://developer.mongodb.com/community/forums/t/mongo-connector/8062
It would be great feature if mongos could be restricted at that level to read from primary.