Luiz H. Rapatão

I'd recommend you to take a look at Java and Kotlin, the first due to the number of companies that actively use it in your products. Kotlin is gaining adept since it is fully compatibly with the Java ecosystem but usually requires less code to do the same (ignoring other benefits of the language). Another benefits of the Kotlin is that it is in fact multiplatform, where you could use the same syntax to code for mobile, web and backend applications. The drawback of Kotlin, is the number of open jobs that exists currently compared to Java, but I pretty sure that it will change in the near future.

There is no problem to keep using node.js for your backend. Keep in mind that you already have expertise in it, so you could focus on development instead of to learn a new syntax/framework. There are good libraries in node.js that could help you in the development (services, validations, integrations, etc) also keeps you with a single language to the whole system. Django, as far as I know, it will provide a solid base for you, but it could be too much for your purpose, also could be more complex than you could need. Java provides to you many frameworks to simplify your integrations also could achieve a good performance. Anyway, I recommend you to follow using node.js, since you already know the syntax/platform.

I don't have an unquestionable opinion regarding your use case. I only trend to pick the MongoDB since it is schemaless avoiding null columns that you not always know when it is used (it depends on the source of the data). The only drawback that I could consider is the query's complexity in MongoDB, sometimes it is a bit tricky, when compared to the traditional SQL queries.

In terms of adoption, community and pre-build libraries, I recommend to use Spring Boot but if you are looking for something lightweight, you probably should take a look at Quarkus and Micronaut. I've been using Micronaut in some projects and I'm really enjoying the development process also the performance that I'm achieving (latency, warm up...). The learning curve is also very smooth when you already know Spring Framework, which is my situation. The drawback is regarding the documentation also the lower adoption/community (low number of pre-build libraries/stackoverflow answers). If you are looking for Jakarta EE compliant, then you should go with Quarkus. As a personal advice, I would recommend you to talk with the team also let the team pick a solution that they are confident to work with.