As a big believer in immersion I don't think there is any other way of learning languages other than immersing yourself in it..
Immersion outside of a classroom context, as an adult, is a terrible way to learn a language on its own. Your exposure to comprehensible context is lower than in traditional language learning methods.
In a classroom setting it can be highly effective, but is still best paired with traditional approaches. Grammar is difficult to teach through immersion, and there's no obvious benefit in doing so. You can spend hours going through various combinations of "y" and "en" pronouns hoping to make it stick. Or you could take a quick pause and explain that, roughly, "y" replaces "à + noun", "en" replaces "de + noun", and move back to immersion for practice.
People often seem to have done reasoning along the lines of "children learn languages through immersion, children learn languages faster than adults, therefore learning is faster with immersion." But that's obviously flawed logic. It's just that people rarely make their argument explicit.
2/19: Started lessons on Italki, roughly once a week
1/19: Started occasionally watching Peppa Pig in French (yes, really)
2/19: Changed my phone language to French
For those that don't know, iTalki is a platform where you can talk to native language speakers. You can join either as a teacher or learner. You can get actual lessons from people or just join to have discussions to improve/maintain your language
(As opposed to traveling to a country and living in it.)
Also note that they switched their phone over to French.