Immersion is thought of as the best technique, do you agree with this sentiment? What alternatives are there?

Speaking from experience, I disagree. Straight up immersion is not always the best way to go. If there are similarities between a language you already know and your target language, then yes, immersion alone will do you well. For example, since I know Spanish and can read books in French, I'd likely be able to go to Italy and have a functional grasp within a week.

Immersion without knowledge, however, is really tiring. Sure, you'll learn a lot more culture, but having some level of competency before going is useful. It will make you more interesting to talk to. For example, there's a phrase in Arabic that you're supposed to say when someone gets a haircut . I've forgotten what it is, but now I know that if I ever live in the middle east and find myself in the above situation, I'll know why they said it, which is useful for memory.

Here's my advice: the better you know how your own language work in theory, the easier time you'll have making analogy. You have to start with analogies with your current knowledge base. Buy/borrow a book on linguistic theories of English.