Here’s a website with those FSI courses I referenced earlier, as well as Peace Corps training materials. This is going to be the boring route. Drill drill drill, but you get good at it.

As a general strategy - on the Omniglot forums a billion years ago there was a method called Listen-Read which I think does wonders for me. You pick a longer book, preferably one you have enjoyed and read already in English. You get a copy of that book in English and your target language, as well as audiobook (let’s go with say, French), then you listen to the audio book in French while reading the book in English, then switch to listening to an English audiobook while reading the French book, then the audiobook in French while reading the French.

Librivox and Project Gutenberg are godsends. I did Candide this way, and part of Les Miserables. This is obviously less immediate fun/dopamine satisfying than Duolingo is, but will teach you to read better than Duolingo will. It’s not great at expressive language - while I can read Proust, my « je voudrais un Diet Coke » was not well received in Paris.

If you have a language in mind I can probably point you in some other directions.

Any good free alternatives?

You won't like the idea but...

What language(s)? Lots of good free resources.

LanguageTransfer.org looks good but I haven't tried it myself.