The best method for learning a new language I have ever come across is a combo of Language Transfer + Anki flash cards and making a serious effort to study them 4-7 times per week.
It’s incredible how quickly I started learning doing this every day for 6 straight months despite not having anyone to actually talk with or any media to consume in the new language.
Eventually, I started taking in person classes for the language, which has helped too, but after 3 courses, I still haven’t come close to the level I had learned via Language Transfer and regular self study. Language Transfer taught me quite advance concepts much quicker than traditional classes did.
I would say I learned more studying Greek in 6 months via that method than I did in 5 years learning Spanish via traditional classes. After an additional 5 years (10 years total) studying Spanish, I’ve become highly proficient. It blows my mind how quickly I’ve been able to learn Greek. The way the language is taught in Language Transfer is so intuitive. I wish it had been around when I learned Spanish.
My experience with Duolingo over the years is learning a pretty minimal amount. It’s like a worse version of Anki. It’s mostly fluff. The gamified concept is clever, but the execution leaves a lot to be desired. If you enjoy doing it in addition to the methods I listed above, then I’m sure that can’t hurt. However, I’d recommend avoiding it and using your valuable time on the methods above. I can’t emphasize enough how remarkable my learning path for Greek has been.
P.S. If you’re really craving a gamified learning supplement, try Clozemaster once you’re at a high enough level of proficiency.