Definitely never too late at all. It can seem overwhelming at first because you've never done it before. But just put in time into it, you will improve and become comfortable with it.

What do you mean when you say the app is finalized? Isn't it always evolving? If you don't write any tests, how are you ensuring any new code added are not breaking anything else (regression testing)? Unless the programming language is so expressive (such as F#), it'd be hard to read the code and immediately understand the business logic. Well written tests help to serve as docs.

Daily Drip has many great resources on learning Elixir, and it's $29 a month l think. Definitely check them out. https://www.dailydrip.com/topics/elixir

When I apply best practices (SOLID principle for example) to my OOP project, I find myself doing functional programming. All those OOP patterns are really trying to help OOP programmers write functional programming code. So is OOP a failure? I'd say yes. http://www.smashcompany.com/technology/object-oriented-programming-is-an-expensive-disaster-which-must-end http://blog.ploeh.dk/2014/03/10/solid-the-next-step-is-functional/

It's like after applying the best practices (SOLID) to your OOP. http://blog.ploeh.dk/2014/03/10/solid-the-next-step-is-functional/ I recommend watching this talk too: https://www.youtube.com/watch?v=1pSH8kElmM4

If you are just getting started, remember that syntax isn't that important. You want to use a language that is syntax-simple but very expressive. I'd recommend using Ruby or Python as a start and work on some really simple projects.

http://www.smashcompany.com/technology/object-oriented-programming-is-an-expensive-disaster-which-must-end

IMO To get a job: Javascript, Python, Php, Ruby To become a better programmer: F#, Scala (functional way), Haskell

I actually recommend Elixir, which looks similar to Ruby in syntax but drastically different when you actually program with it. It's a functional programming language that's easier (than Haskell or Scala IMO) to get started with.