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In my experience (using Dynamo for 4-5 use cases while Postgres for many more) a properly configured HA Postgres with RDS costs way more than Dynamo for the same workload. There is only a big catch, data modeling is not as straightforward with Dynamo. You cannot change it up so easily than with Postgres and your access patterns must cover every single use case. If you can do that, model your data and the access pattern fits Dynamo great, if not you are going to have a hard time.

Managed Postgres still needs to be restarted for config changes and instance resizes for example.

Managed postgres services tend to be fairly expensive for production usecases at any small - medium organization, and they all come with their own little caveats here and there.

Are you running one cluster per application? I've seen that a lot but it's actually quite inexpensive when you use one cluster for multiple applications.