the businesses without the upgrades are slow to get money- you can't buy the goods basically and must do the missions to resupply. I would advise grinding that and the premium missions/events until you get the sub! Once you have the sub you can grind the heist (even without a sparrow) in about 2 hours. When you get the sparrow this will drop to 50-60 mins per run.
Arms will get you the most - make sure you sell after it gets a quarter full so that you only get one vehicle to run. roughly 140k in stock. You could save up and buy the upgrades but it will take longer to eventually get the sub.
maybe i do have some upgrades then because i can buy supplies @ 75k
anyway im going to work on that
whats the premium missions/events? is that just the ones that are 2x?