Last time I was scaling a monolith I basically did it purely by splitting up the DB into multiple ones with different purposes and improving query perf just through that.

I would've had to do all the same work to extract it to microservices, but also would've had to do a lot more on top. Had it not been a legacy system, probably worth it - but as it was, leaving it as a monolith querying a now-split-up set of DBs was cheaper and faster.