etrotta

Like others said, definitely consider using Jupyter Notebooks for development (note: not necessarily JupyterLab, just Notebooks - multiple IDEs like VSCode support them).

For an actual GUI, I would recommend considering some web dashboard frameworks like Streamlit, Dash or Gradio if you are not felling like spending too much time organizing the layout and making it pretty.

Other than that, I would also recommend taking a look at a few plotting libraries other than matplotlib like Bokeh or Plotly.

Would an experienced Java developer that has never touched JavaScript before know how to use JavaScript well enough to use it in a professional application without having to learn/revise a lot of things?

Would an experienced JavaScript developer that has never touched Java before know how to use Java well enough to use it in a professional application without having to learn/revise a lot of things?

No and No - They are like car and carpet. Unlike something like C and C++ or JavaScript and TypeScript that you can easily adapt to in a relatively small amount of time, Java and JavaScript serve completely different use cases and require significantly different skillsets.

USA paying high salaries = "Europe pays so bad"... You realize that there is more to the world than just USA and Europe right?

You realize that they are only able to pay for "what’s historically been free" because of advertisements right? Google might be able to sustain Youtube even without ads because they have other revenue sources, but the vast majority of their revenue are from advertisements, and it would be a massive loss of money to keep Youtube up without it generating ad revenue. Hosting videos is one of the most expensive things a website can do. If we are to ever hope for other companies to compete with Youtube, we should expect for it to not be free. All that said, Google can still go fuck themselves though - I cannot possibly endorse their methods.