Transactional databases are not really the best tool for writing tons of (presumably) immutable records. Why are you using it for this? Why not Elastic?
The total number of users in your system is not a performance characteristic. And transactions are generally wrong for write-heavy anything. Further, if you can just append then the transaction is meaningless.
Most systems are based on the number of users performing operations on the application. Majority of people on HN never work on anything with more than 100k users, yet they introduce mountains of infrastructure and blame cloud for being expensive when they never needed that infrastructure to begin with.