Re: transitions, just copy what you hear in popular beats on actual songs.
I usually sample, but when I compose, I usually start with 1 melody and then after 4 bars bring some percussive elements and the bass, 4 bars later add a countermelody, then transition into the hook with a snare roll or sound effect and/or drop. Don't over think transitions.
Bass needs to hit harder, that shit sound make the trunk rattle but it sounds barely present in the track imo. Really the like the lead melody and the countermelody was ok.
This track has potential but just needs the arrangement fixed. Good job.