I'll be frank, if OP is making six figures gross right now, the opportunity cost of going to law school is probably heavily on the "don't go" side. Trading 3 years of 6 figure earning power while young to go dump cash into an institution to get out and make.... six figures in the best case is lunacy. Take the money you'd spend on law school and sock it away in the market and be content you're likely going to come out ahead.
OP is asking about sole practice. In such a case, there's no minimum income, and it cannot be related to what someone might pay you as an associate. You earn whatever you earn, which could potentially be very little, yes. Doesn't have to be, but it's a real concern not to be dismissed lightly.
I'll try to post something intelligent soon. In the meanwhile, OP, start by reviewing this topic. It's going to answer at least some of your questions.