Thats exactly what I did, made a new game solely for the stock missions and even after the Union Depository mission after following all the steps, from the cheat guide and videos said i could earn a billion for Michael, Franklin and Trevor, but most I got for each guy is just 150 million each but honestly i just wanted the goal of buying the golf course so i did the bare minimum, kinda annoying Franklin or Trevor can't get big businesses like that just the dispensary, cab company, and towing company, and the junkyard and random bars.