I'm not much of a singer, so I won't pretend to offer technical advice. Here are my reactions. I listened to the first and last of the performances you posted. You can sing. You seem to have good control over your voice. Pitch sounds pretty good for acapella. It would be worthwhile to get some lessons to help develop your talent. On the parts where you belt it out, your voice takes on a harsh edge. This could be partly due to the recording, but I think there is an issue that you could fix through training and practice. I noticed on the first song, particularly, that it was difficult to follow the lyrics. This should not be happening, especially on acapella. Work on diction and putting the lyric across, which in popular genres is just as important as the melody.
This is my personal preference, but I could do with a lot less of the vocal embellishments--the breathiness, the trills, etc. These vocal mannerisms have gotten completely out of control in the R&B genre especially. Go back and listen to Otis Redding's original version of Dock of the Bay. There are a few "ohs" and "yeahs," but for the most part he's just putting the song across in a way that is honest and soulful. He doesn't have to embellish much, because the song is good and so is he. This vocal honesty is timeless. I predict we will be listening to Otis Redding long after we have stopped listening to Whitney Houston or any of the current crop of R&B singers with their autotuned vocal gymnastics.