I will have to leave this to someone who’s done this before. I personally don’t think you can be reactive with two different DBs based on Oplogs.
I hope not to mislead here… You could look into observers which is basically the link I shared with you earlier or here: ([Solved] Meteor observe not working properly in 1.6) or since you use the Mongo Driver, perhaps directly Mongo tech like https://docs.mongodb.com/manual/reference/method/db.collection.watch/. If not too critical I’d go with polling. Another way is to keep a “trigger” on your main DB if you need to update based on Inserts. Example:
I save all teams and team members in a “secondary” DB.
In my main DB I keep a collection of the number of team members in a team. (When I write a team member to a team in DB2 I also increment the counter in DB1.) I subscribe to DB1 (for which I use the Oplog). In React, with a useEffect() dependent on the counter, I pull data if the counter changes. I never observe/subscribe data sets. In most cases for Insert/Remove records it is so much cheaper to observe counters in counters collections. In case of Updates of documents it is a different story :).