Full disclosure, I helped review a couple of the drafts. (We don’t have any formal relationship, we’re just friends–although we do integrate VividCortex with Datadog for our customers).
My only suggestion for monitoring RDS+MySQL above and beyond what Datadog wrote is that query monitoring is vital. Your databases exist to run queries, so you really ought to monitor their performance! However that doesn’t really make sense for Datadog, since it isn’t a query centric monitoring platform. You need a tool like VividCortex for that.
Datadog’s blog also has a bunch of great content on monitoring other popular products too. Subscribe–it’s worth it. I’ve been a subscriber to their blog for years.