The way to grind billions is a hundred million at a time. The way to grind a hundred million at a time is with a cargo ship taking hauling missions in stations where you are friendly/allied with most of the factions. Preferably as part of a trade loop with a station with which you are also allied/friendly with most of the factions. I go to my favorite refinery station, take missions for extraction goods, and fill my cargo hold with silver. Then I go to my favorite extraction station, sell the silver for 10 mil profit, buy the commodities I need for the original station, and check the transportation missions becaus there will be occasionally be a 40+million mission for less than 100 tons of palladium or tritium. Rinse. Repeat. And repeat. I personally never take a mission to haul more than about 3 runs' worth of cargo. Preferably less. I don't stack up too many missions, either, or the interdictions from mission enemies will kill my pace. If you don't already have the reputation, you will have to spend a few hours grinding your way up. You can average a hundred million an hour with the occasional 30-45 million bonus for an easy tritium or palladium mission. Half a billion in an evening of playing. Billions per week.
Bounties are slower. The time to kill, the time for repairs... they kill the pace for me. Exploration and exobio make big money too, but depend a little on luck. Hauling is a steady 100M per hour plus bonuses if you set everything up right.