- dismissing DynamoDB unless you're doing "just key/value pairs" is IMO too reductive. You can go a long way with DynamoDB, and for smaller tables it has the most bang for your bucks (plus it's dead easy to set up and configure)
- building and maintaining your own DB cluster should not be your first move, even if you think you need "a high level of engineering". Start with a managed RDS instance and only if you reach its limits should you consider rolling your own (unless, of course, you already have an exquisitely tuned DB that you need to migrate to AWS. But honestly, as long as you're migrating, shouldn't you take the opportunity to move to something with less maintenance?)
- managed RDS is fine, but, once again, DynamoDB will frequently be cheaper.