Save Lester's assassination side missions until you finish the game all the way, you can max out your cash that way. It can actually become impossible to 100% if you don't save, then invest properly. Some businesses you can buy are too expensive unless you really juice those missions. I'll dm you the guide I use to max out gains. You only have to finish the first assassination mission as part of the story, the rest you can save til the end when you get your final heist payout.