Multi-tenancy at scale with RDBMS is just hard to do. Once you get to certain volume, you just have to have different data stores for different needs. Only a subset of business require ACID properties so you would use RDBMS for those and for the rest choose a NoSQL stack that has the least resistance to ramp up WRT team skill set.

You can get away without using NoSQL but you probably would end up spending time and money maintaining several DB clusters, migrations, handling replication lags etc.