> Split up the monolith into multiple interconnected services, each with its own data store that could be scaled on its own terms.

Just to note: you don't have to split out all the possible microservices at this junction. You can ask, "what split would have the most impact?"

In my case, we split out some timeseries data from Mongo into Cassandra. Cass's table structure was a much better fit — that dataset had a well defined schema, so Cass could pack the data much more efficiently; for that subset, we didn't need the flexibility of JSON docs. And it was the bulk of our data, and so Mongo was quite happy after that. Only a single split was required. (And technically, we were a monolith before and after: the same service just ended up writing to two databases.)

Ironically, later, an airchair architect wanted to merge all the data into a JSON document store, which resulted in numerous "we've been down that road, and we know where it goes" type discussions.