I work for a foreign language instruction services company. We mostly serve government and military, but we do accept private students. All of our linguists are extremely skeptical of apps like Duolingo. And before anyone replies something like "your company has a vested interest in propping up instructor-led language training", if apps like Duolingo actually worked, the government--especially the military--would stop paying us in a heartbeat.

There is a lot of diversity in instructor-led training. None of our classes are anything like what you would have experienced in highschool or college. Our largest classes have 4 students in them, and that's strictly a cost-cutting scheme that Gov has insisted on for maintenance training. You'll be one-on-one with your instructor. They'll be a native speaker with often decades of experience teaching language. They'll be tayloring a lot of supplemental material specific to you. We even have some of our own apps that we've developed (that's my job), nothing like Duolingo (though nothing you can just use without our curriculum or instructors).

A lot of people say "the best way to learn a language is to live in a country where it is spoken". Clearly, that is unrealistic for the vast majority of people. I would also say it presumes you are a specific kind of extremely extroverted, fearless person. Most people find themselves becoming quite introverted in situations where they can't communicate easily.

For these reasons, I think the most realistic answer for the best way to learn language is private tutoring. The OP take about having used Duolingo for almost 10 years. That seems absurd to me. I would hope they have mastered at least half a dozen languages with that kind of effort. Language training shouldn't take that long.