I'd recommend to learn Spring as it is very widespread in the industry and provides a lot of easy integration into most of the common backend tech stacks. Rather than learning Java you could look into Kotlin. It's a very consistent, stable and well-thought language in my opinion and not as verbose as Java. Many problems can be solved with Kotlin in a clear and elegant way while also always having the option to use data structures and libs in JVM. It is also has a very good support in Spring.

Why not pick Django or Flask (both Python)

I have worked in a Spring environment for many years and I still love working with it. Super quick to get a base application running and get coding, thanks to Spring Boot's easy and straight forward integration with Tomcat. However, I will try and answer this question from another perspective: look at topics such as popularity of the language, average statistics on community contribution to their repositories and hiring availability from companies. If you are going down the API route for backend, leverege on your experience in the Node world by looking into ExpressJS (or even NestJS). The JavaScript world is really excellerating at a lightning speed, and I could recommend exploring those worlds a bit more, should it be a comfort level for you. However, my biased answer is tryout Java, followed by Spring afterwards. No disrespect to any .NET developers out there, as there are a few topics in there which are fantastically implemented.

IMHO I think it is the best framework to build software on. Since you are coming from Java, It should not be too difficult to adjust to C#. NET Core has come a long way. NET6 is just amazing. With the minimal API, https://docs.microsoft.com/en-us/aspnet/core/fundamentals/minimal-apis?view=aspnetcore-6.0 REST API should be very easy work for starters. When you settle in, you can go more advanced.

Although this is dated - https://download.microsoft.com/download/D/E/E/DEE91FC0-7AA9-4F6E-9FFA-8658AA0FA080/CSharp%20for%20Java%20Developers%20-%20Cheat%20Sheet.pdf it provides you side by side comparison of syntax. Another cool read is - https://betterprogramming.pub/java-to-c-c-to-java-f766c9f659c4

Feel free to reach out if you need any assistance.

Welcome aboard

You will find that the transition from Java to C# is pretty smooth. Working with .NET has become much more intuitive and has a lot to offer. I highly recommend using Visual Studio as your IDE. It makes things much easier and is not too different than other IDEs like IntelliJ. There are also Visual Studio templates for web APIs that make it really easy to get up and running. Microsoft has good documentation and tutorials to help you get familiar with the technology. I would start there.

I would recommend that you become familiar with the C# language. J. Albahari's reference book is suitable for this. To learn rest api development, read Andrew Lock's book "Asp Net Core in Action"

If you are already committed to Python for the data analysis then it would make sense to use Django for the framework and stick with Python throughout. In general the fewer technologies you use (unless there is real justification) will improve process and long term costs.

All things being equal the fact you would not have less (likely no) licensing costs with a Python based solution will overall reduce your long term costs.

If you will be doing data analysis and complex algorithms Django is best suited for this because of it' maturity and the huge communities providing lots of Python libraries for data analysis and more.

Since the course is in Python, a Python web framework seems like a logical choice. Django is a good option, but Flask and FastAPI are far more lightweight than Django, and great for quickly adding an API on top of your ML/NLP code. Django has its merits, especially the generated admin interface, but requires a lot of boilerplate code.

The one thing I wouldn't do is mix the two, different structures and ideas for how to do things

Taking a class in python already? I'd lean to Django Want to learn a language that will carry forward to a good paying job .net

Python is an excellent language, though it's often being replaced with things like go which uses a similar syntax

Over time you're going to learn many languages if you want to be a great programmer spend your time with one and dive deep, learn what makes it shine and what the draw backs are grow proficient with it and then consider learning something different so you can compare

Early on learning a single language at depth is more important than tinkering with several or learning any specific language

Good luck