So I’ve been drop thumbing lately. Everything has been going well but I’ve noticed the backside of my thumb usually touches the string above it, ever so slightly, but enough that it would be muted if it was ringing. So far it has not been an issue but i don’t know want to to become one down the line. I’d rather nip it in the bud now. However, I’m a big dude and I find it near impossible without twisting my hand to grab it with the tip of my thumb. Since I’ve never seen anyone do this, I thought I’d ask if it was even a big deal.
This is one of those questions which make me actually look at how I do things in detail. Seems I don't really use very the tip of my thumb but more the side of the tip. If I've got the banjo sloped a bit towards me (which I'm doing because I'm trying to see what's actually happening) I do sometimes lightly touch the next lower (in pitch - i.e higher up geographically) string with the bit of my thumb a bit behind the striking area. Seems to have little or no effect on the sound I make. As you say "It would be muted if it was ringing". But in a typical drop thumb "roll" it's a beat or so since you sounded that string - So it probably isn't (much). I wouldn't worry too much.
Accuracy will come with repetition and practice. I find a metronome helps keep me at a good practice pace. It may take a year or two to develop effortless accuracy. That was my experience when learning many years ago.
The part of the thumb that strikes is the side flesh, about where the quick of the nail ends, at roughly 45 degrees angle to the string. I don't avoid the other strings at ALL - indeed I drop it right in between (at least far enough to get the right attack) as I make the note with my index or middle, then "pluck" (thuck? plumb?) as my hand continues its post-strike motion. It's OK if it mutes the string above. Indeed I sometimes feel the upper part of my thumb on those lower strings as well.
This is something I mention often btw: the thumb ("should" is way too strident) almost always lands at the same time a finger strikes. This happens whether noting a string, a brush whatever, and happens when thumbing the fifth string or another string. Since it works with the natural motion of the hand, it's way more precise than making it flail around and hunt for the string independent of the fingers, and the thumb is following the momentum of the hand so the hand can better keep its "claw" shape the whole time. Makes playing accurately at speed much easier.