We build a Factoring-Software for a company in germany and realied on PostgreSQL as main banking and transactional database. Works flawlessly. It's very fast, realiable and well documented. It's a very good product.
My advice for your current needs would be to take a look at https://hasura.io/. I like the service and the fact that it's open source, too. It shippes with PostgreSQL database and gives you the power to map your database relations to a GrapQL endpoint. GraphQL works really well with a React frontend. Hasura getting started docs are well written and they also provide nice use-case and hands-down coding tutorials to get you started. Check if it matches your needs. Here are the features: https://hasura.io/all-features Here is their tutorial to kick it off: https://hasura.io/learn/graphql/hasura/introduction/ Here's an article about scaling their service for GraphQLto 1 million active subscriptions (live queries): https://hasura.io/blog/1-million-active-graphql-subscriptions/
Have a nice day! Cheers, Chris