Hello,
Fellow mandarin learner here, I have been learning mandarin for the last three years, using a variety of apps. Duolingo is good, but the vocabulary is limited and you don't get to practice the characters. Falou is another good app that is good for conversation.
I have also used the HSK apps, there is one for each level (1 - 5), and the HSK Study and Exam - SuperTest app which is fantastic for listening, reading, and writing.
The biggest help for me however has been paying for language lessons with a company called Darling Mandarin, (
https://www.darlingchinese.com/index/hsk.html ). the lessons are very well organized, and you learn at your own pace.
Hope this helps!