Simply Piano was pretty fun but the Android version is lacking in features. I connected a Roland FP-30 to my Samsung Note 10 using a OTG adapter and it works just fine. I also had some issues getting audio without using the phone speakers since there is no minijack and Bluetooth latency was unacceptable.

I tried Playground sessions and Flowkey only briefly but I liked the UI in Piano Marvel better. Also Piano Marvel doesn't require your credit card to get a free demo.

I ended up with Piano Marvel and a Windows laptop. What I like is that it's just sheet music. Very good for training rhythm and tempo. The UI is a clunky and buggy SPA, but it works. Biggest con is that the music is pretty bad. Simply Piano has licensed tons of popular music while Piano Marvel has mostly free and classical music.