My guess is it will just take some more conscious effort and practice. This is a very common issue though many people have (myself included).
Instead of trying to “quiet down” your thumb, you could try the opposite approach for a while, bring up the volume of your index and middle finger to match your thumb.
Once you get all three fingers the same volume you could then work on turning the overall volume “up or down.”
It’s hard to say without seeing the banjo but it’s possible you’re hitting the string too hard which is making it sound “sharp.”