This app was mentioned in 3 comments, with an average of 2.00 upvotes
There's Noah's Bottle https://play.google.com/store/apps/details?id=com.wen.NoahBottle
The generated beats feel fairly accurate, and there's technically no limit to songs that you can put in (you just have to delete the song in a slot, but it does erase your high score for that song, which isn't too much of a deal for me). The game does have video ads though, and they can pop up even if you are offline but have previously booted up the game while you're connected to the internet, but from my experience they only appear after you complete a song.
The IAPs are for extra song slots (so you don't have to keep deleting and re-importing songs in your list, but again it's not really an issue and you can add any amount of songs in the game if you don't mind scores being deleted), ad removal and a currency that can be used to unlock cosmetic stuff like beat shapes and sounds, but this currency can be farmed as you play.
Here's some cool games I had a blast playing over the years.
Note: All these games are free but lost in harmony has a iap to unlock the full game -
Full of Music 1 MP3 Rhythm Game