If you're doing it solo then you don't need to scope out the whole Island as you can only carry 2 lots of secondary loot, you'd end up wasting a lot of time scoping out things you can't take,
just head straight for the tower and hack the compound camera's to find your main target
it'd be worth watching a few guides on Youtube
Found a decent vid by TCG , Got it down to 15 minutes
Went with Sub for entry via drainage tunnel , Kill the minimum guards i have to , grab the primary objective and the contents of the office safe , and then out and over the cliff into the water to swim away.
Averaging 1mil take each time and have gotten the prep missions down fairly well (Destroying the buzzards is a piece of cake now i've upgraded the sub to have missiles) Only one i find tricky now is collecting the weapons if its the Schlongberg building as those heli's coming at you on the roof are non-stop (and massively overpowered)
If GTA$ are now this easy to come by , I guess that GTA 6 (Or whatever the next one is called) must be on the cards within the next 12 months? I can't see that they'd lose the income from shark-cards so willingly otherwise?