I think I got to 4 furthest. I saw a youtube video with a guy at a deeper level and there was a trader. I think it’s just more dangerous and expensive to make them drill further.
I think a good system would be for it to have a cooldown given time is money (or rather food wasted) in this game, and when calling somebody down the further you are the higher the chance of some malfunction and the chance of the malfunction being more severe, so that you would have to waste a bunch of medicine rather then find a very specific item.