I recommend playing 15/10 and using the Game Review function to see where you went wrong. I found it's best to deep dive into each game and thoroughly analyse for 30 minutes.
The 1400-1600 level is about as high as you can get by just playing. Getting higher requires study and work. Another thing is that as you improve, further improvement gets harder and harder, which is why your progress is slowing down.
I am playing chess since 3 years and my rating is stuck in rapid between 1350-1410. I play really insane games sometimes which analysis shows rating of 1900 but i really struggle to even push my rating out of 1420 . I need some valuable insights and suggestion and on what things should i work on. I play caro kan, ruy lopez and king's gambit openings.