I can't speak to your specific use case, but I can tell you that a relatively small RDS instance is probably a lot more performant than you think. There is also "Aurora Serverless" now which I've just started to play with but might suit your needs.
As far as what makes Dynamo a good fit, I almost take the other approach and try to ask myself, what makes Postgres a bad fit? Postgres is so flexible and powerful that IMO you need a really good reason to walk away from that as the default.
Aurora wasn’t allowed at the time. The system is a simple stream logging app. Wonderful for our use case. Dynamo for so far. Corp politics made the RDS instance annoying to pursue.
As far as what makes Dynamo a good fit, I almost take the other approach and try to ask myself, what makes Postgres a bad fit? Postgres is so flexible and powerful that IMO you need a really good reason to walk away from that as the default.