Pro Suggestion: I've been in the studio with a number of upper level rappers (as a studio engineer) and a common trend is I see them practicing their bars without the beat playing. This way they're properly executing their tonal cadence separate from the beat, and that way, when the beat is playing, they can more easily fit it to the beat. Sometimes if a beat is playing, it can make it difficult to get your voice sound good because your brain is partly distracted by the beat and not fully focusing on the voice.
Here's one such example of one of Europes biggest rap artists (it's in German, but you can see the point):
Rehearsing his verse without beat: https://www.youtube.com/watch?v=bNtwxCoGgLA
Recorded song: https://www.youtube.com/watch?v=JsYPXCoVrdc
Without understanding him, you can clearly see that when he rehearses his verse, he is doing it without the beat playing so that he can get the tonal cadence correct. When he records the actual song, he is a little bit more intense with his vowels and such, but the overall cadence that he rehearsed is the same.