Duolinguo feels more like a very annoying ad than a learning tool, often under-explaining what it tries to teach while heavily penalizing and shaming the user for getting things wrong.

What I would suggest, other than hiring an actual teacher, would be to immerse yourself in works from their language, such as music, comics, books, movies, and so on. Also, Spain's Real Academia Española has a dictionary+conjugator site that is VERY helpful, even for "dialects" such as the Latin America variations (link for the tool is dle.rae.es). Finding someone to try to talk to in Spanish may also help, although getting to know some of the grammar beforehand is advised.

Now, as for either free or FOSS programs specifically for learning the language, sadly, I'm not aware of any.

As far as apps, people sometimes use Anki and get a card pack (commonly the most used words or phrases), but that is more of a supplementary tool.

Duolingo also has a website, if installing the app is an issue. Just use adblock in this case.

I took a year of ไทย classes after work & that was enough to go out & start conversations with strangers. Classes where they speak your native tongue can be invaluable since you can ask very specific grammar questions most native speakers don’t how to answer & the class puts a social pressure on you to keep up. Speaking with natives & listening to media can help a lot.

I just web searched for 'librivox spanish', and what do you know, there are 34 pages of Public Domain spanish language books to choose from and play as background sounds to your life.

To help hear real world examples of inflection, intonation and pronunciation, and pick it up passively.

https://librivox.org/search?title=&recorded_language=5&sort_order=alpha&search_page=1&search_form=advanced

Do you happen to live near a community college? Or is that not what you mean by on your own?