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Realistically very few solo developers are able to make sellable products.

Even fewer make a net profit. You're probably going to lose money here unless you want to work for a corp.

I'd go with Python or JavaScript. It depends on your first project. Python is basically very good with data science and okay at everything else. JavaScript is almost the only way to build a website and okay with other things.

Write something simple. Like a sales tax calculator with Python. Then slowly build yourself up to other more challengingly things.