As you no doubt know, many great players wear a thumbpick, and many great players don’t. I used a thumb pick for some years early on, and now use one only occasionally.
Some reasons to use one:
1) the angle of attack with your right thumb (assuming you are right handed!) may be very low – nearly parallel to the strings – especially if you are trying to mute the bass strings with the base of the thumb/palm, like Chet Atkins. If you use this low hand position, a thumb pick protruding at a right angle from the thumb provides a good attack angle for the bass strings;
2) for many people a thumbpick provides a louder, more focused attack than the skin of the thumb and the thumbnail;
3) you can use it as a flatpick in the middle of a piece.
Some reasons not to use one:
1) No click noise from a thumbpick picking a string;
2) Tone in the bass matches tone in the treble;
3) You can still play if you don’t have a thumbpick with you!
What kind of tone you want, and what music you want to play may have some bearing on your decision. If you want to play like Chet Atkins, then by all means wear a thumbpick. But English virtuoso John Renbourn did a great job of muting bass strings without one.
I have watched Pierre Bensusan, the great French fingersylist, wear a narrow thumbpick near the thumb knuckle. By changing the distance of his palm from the strings – and, hence, the attack angle of his thumb – he could pick with it, or without it in the same piece.
I have also watched my late friend John Renbourn get the muted-bass-strings Chet-Atkins sound without a thumbpick. You can manage this by muting the strings near the bridge with the base of the palm nearest the little finger, turning your forearm slightly, and making sure the lowest knuckle of your thumb (connection to the palm) is about an inch off the strings. That way you get a good enough attack angle with the end of your thumb.
As far as thumbpicked notes being louder than the finger notes (without picks), your ear will tell you when the treble and bass need to be better balanced. You balance them by relaxing the thumb a bit and plucking the bass notes softer than the finger notes, or by muting the bass strings with the picking hand.
Classical players (almost) never wear a thumbpick, but their picking-hand position is higher off the strings than most thumb pickers. That way, classical players get a great attack angle (as high as 45 degrees, perhaps) with the thumb, and get plenty of tone and volume from skin and thumbnail.
As for me, these days I only use a thumbpick when I stand (the guitar angle is different than when I sit, and I rest the base of my palm on the bridge for stability), or when I play tunes like Mancini’s A Shot in The Dark, which has a muted but punchy bass line. I almost never use one when I sit.
You’ll have to find what is comfortable for you. After some time and experimentation, you will have a better idea of whether or not to use one, and, if so, what kind. Good luck!