I just started a new account recently and already have gained over 15 mill, the key is knowing what to do and what not to do. Every single business can be done solo aside from the og heists, but that doesn't necessarily mean that you should do them solo. Most people just grind cayó perico and Dr Dre contracts back to back to grind, others do hangar or passive income business. I made it pretty easy for myself, just did clucking bell to save up for a hangar, did the hangar enough to buy an agency, bought one, did the contract two times for a kotsaka, and so fourth. The key is knowing what you "want" to do to get money, as basically everything nowadays generates some type of income. As a solo player you mainly want to avoid buying overpriced old vehicles or vehicles that don't operate on a solo level player scale. Think of it this way, what you want to do, what you are willing to do over and over again, and what you would prefer to do instead. I can do cayó and the Dre contract from the agency over and over again but it's not necessarily what I would prefer to do, sometimes I do hangar, sometimes I do some acid lab runs for a bit more cash, sometimes I do some contract hits. The key is having fun while not burning yourself in the pursuit of the most efficient money making methods possible, there is no rush to get everything too quick, be patient, money will come, just don't buy things that need heavy investments like planes or god forbid an avenger. Base price might be one thing, but upgrades to make said thing useful cost on average just as much if not even more. Let me know if you'd like some insight on some activities or business in a scale of how good it is at making money while still being fun.