The only thing I know that works for sure is getting an empty public lobby and grinding the motorcycle gang and CEO businesses.
You just go to a public lobby with people in it. Go to your Xbox network settings and do the test network connection thing (whatever it's called), let it finish, then back out, and you're in a public lobby by yourself where you can do your CEO thing and fill up your warehouses and sell your shit off, unbothered (a person might spawn in later, but they usually leave).
You can move your shit solo (takes a long ass time), or you can invite your friends to help you move shit quicker.
Just keep your warehouses supplied, and let the money grow.
When I transferred over from X360 to x1, I ended up getting back over 2 Bil from the modded money and challenge racing days. I still fill up my warehouses just in case Rockstar ever comes for that money. I don't even really buy shit anymore, anyway. I'm just paranoid like that.