Wouldn't you also lose all the resources processed by the FOB during that period? I guarantee you the potential losses from invasion pale in comparison to that.Under your steam userdata folder is a folder called 311340 that has your MGSV saves in it. After you close the game and Steam finishes its sync, make a backup of 311340. Next time you start the game, if you have been invaded, quit the game and restore that folder. The catch is you have to remember to back up that folder every time you quit the game.
So what I do is log on and check if I've been invaded. If I haven't, I disconnect from the network and play. When I quit, I make a new copy of 311340.